Chapter 6 Application of Derivatives (Concepts)

Derivatives as a Rate Measure

The concept of a derivative is the cornerstone of calculus, acting as a mathematical bridge that connects static functions to dynamic change. In various disciplines such as Engineering, Sciences, Social Sciences, and Economics, we rarely encounter systems that remain constant. Instead, we observe quantities that evolve, fluctuate, or respond to shifts in other variables. The derivative provides a precise, quantitative measure of this responsiveness, known as the rate-measure.

---

The Mathematical Foundation of Rate of Change

To understand the derivative as a rate measure, we must distinguish between the average rate of change and the instantaneous rate of change. Consider a function $y = f(x)$, where $y$ is the dependent variable (the effect) and $x$ is the independent variable (the cause).

Average Rate of Change

If the independent variable $x$ changes by a small amount $\delta x$, the dependent variable $y$ will subsequently change by an amount $\delta y$. The average rate of change is simply the ratio of these two changes over a finite interval.

Instantaneous Rate of Change

In most scientific applications, we are interested in the rate of change at a specific point rather than over an interval. As we make the interval $\delta x$ smaller and smaller, approaching zero ($\delta x \to 0$), the average rate transforms into the instantaneous rate of change.

Mathematically, this is the definition of the derivative:

---

Elaboration Across Various Disciplines

The utility of the derivative as a rate measure extends far beyond pure mathematics. Below is an elaboration on how this concept is applied in different fields:

A. Physical Sciences and Engineering

In physics, the most common application is Kinematics. If $s$ represents the displacement of a particle and $t$ represents time, then:

1. Velocity ($v$): The rate of change of displacement with respect to time.

2. Acceleration ($a$): The rate of change of velocity with respect to time.

B. Economics and Commerce

In the context of Indian trade and industry, businesses use derivatives to optimize production and pricing. This is often referred to as Marginal Analysis. For instance, if $C(x)$ is the total cost in $\textsf{₹}$ for producing $x$ units of a commodity:

Marginal Cost (MC)

It represents the additional cost incurred by producing one more unit. It is the instantaneous rate of change of total cost with respect to the output quantity.

C. Social Sciences and Biology

Derivatives are used to model the growth rate of populations. If $P$ is the population of a region at time $t$, then $\frac{dP}{dt}$ represents the rate at which the population is growing or declining at any given instant.

---

Geometric Interpretation

Geometrically, the derivative $\frac{dy}{dx}$ at a point $P(x_0, y_0)$ represents the slope of the tangent to the curve $y = f(x)$ at that point. This slope tells us how steeply the "output" variable $y$ is rising or falling for every unit increase in the "input" variable $x$ at that exact location on the graph.

If $\frac{dy}{dx} > 0$, the quantity is increasing. Conversely, if $\frac{dy}{dx} < 0$, the quantity is decreasing. If $\frac{dy}{dx} = 0$, the quantity has reached a stationary point, such as a local maximum or minimum.

---

Definition and Derivation of Rate of Change

To establish the derivative as a mathematical tool for measuring change, let us consider a continuous function $y = f(x)$. Here, $x$ is the independent variable (such as time, length, or quantity) and $y$ is the dependent variable that changes in response to $x$.

The Concept of Increments

Suppose the independent variable $x$ undergoes a small change, denoted by the increment $\delta x$. Because $y$ is a function of $x$, this change produces a corresponding increment in $y$, denoted by $\delta y$.

To find the change in $y$ alone, we subtract the original function $y = f(x)$ from equation (i):

Average Rate of Change

The average rate of change of $y$ with respect to $x$ over the interval from $x$ to $x + \delta x$ is defined as the ratio of the change in the dependent variable to the change in the independent variable.

Derivation of Instantaneous Rate of Change

The average rate provides an overview over an interval, but in physics and engineering, we require the instantaneous rate of change—the rate at a specific, exact moment. To find this, we allow the interval $\delta x$ to become infinitely small, approaching zero.

Taking the limit as $\delta x \to 0$ on both sides of the above equation:

By the definition of the derivative in calculus, this limit is denoted as $\frac{dy}{dx}$ or $f'(x)$. Thus, the instantaneous rate of change of $y$ with respect to $x$ is:

If we wish to calculate this rate at a specific point, say $x = x_0$, we denote it as:

---

Related Rates and the Chain Rule

In many real-world scenarios, we encounter situations where two or more variables are not directly linked but are both dependent on a third variable, most commonly time ($t$). For instance, as the radius of a spherical balloon increases over time, its volume also increases over time. In such cases, the rates of change of these variables are "related" to each other.

Mathematical Derivation

Consider two variables $x$ and $y$ that are expressed as functions of a parameter $t$. Let:

To find the rate of change of $y$ with respect to $x$, we apply the Chain Rule of differentiation. The Chain Rule states that if $y$ is a function of $x$ and $x$ is a function of $t$, then the derivative of $y$ with respect to $t$ is:

By rearranging the terms in the above equation to isolate $\frac{dy}{dx}$, we obtain the formula for related rates:

This derivation shows that the instantaneous rate of change of $y$ with respect to $x$ is the ratio of their respective rates of change with respect to the common parameter $t$.

---

Geometric and Physical Significance of the Sign

The sign of the derivative $\frac{dy}{dx}$ is crucial for understanding the behavior of the relationship between $x$ and $y$.

1. Positive Rate of Change ($\frac{dy}{dx} > 0$)

If $\frac{dy}{dx}$ is positive, it indicates that the variables $x$ and $y$ move in the same direction. That is, as $x$ increases, $y$ also increases. Conversely, if $x$ decreases, $y$ also decreases.

Example: In a circle, as the radius $r$ increases, the area $A$ also increases, so $\frac{dA}{dr}$ is positive.

2. Negative Rate of Change ($\frac{dy}{dx} < 0$)

If $\frac{dy}{dx}$ is negative, it indicates that the variables move in opposite directions. As $x$ increases, $y$ decreases. This is often seen in problems involving distance (e.g., a ladder sliding down a wall).

Example: If a vessel is being emptied, the height $h$ of the liquid decreases as time $t$ increases, making $\frac{dh}{dt}$ negative.

---

Commonly Used Formulas in Related Rates

Competitive exams often feature problems based on geometric shapes. The following table summarizes the formulas where derivatives are frequently applied as rate measures:

Shape	Variable (y)	Formula	Derivative (dy/dx)
Circle	Area ($A$)	$A = \pi r^2$	$\frac{dA}{dr} = 2\pi r$
Sphere	Volume ($V$)	$V = \frac{4}{3}\pi r^3$	$\frac{dV}{dr} = 4\pi r^2$
Sphere	Surface Area ($S$)	$S = 4\pi r^2$	$\frac{dS}{dr} = 8\pi r$
Cube	Volume ($V$)	$V = x^3$	$\frac{dV}{dx} = 3x^2$
Cone	Volume ($V$)	$V = \frac{1}{3}\pi r^2 h$	Variable (depends on $r$ and $h$)

Important Remarks

1. Unit Consistency: Always ensure that all quantities are in the same system of units (e.g., if the radius is in $cm$ and time in $seconds$, the rate must be in $cm^2/s$ or $cm^3/s$).

2. Instantaneous vs. Average: If the question asks for the rate "at $x = 5$", it refers to the instantaneous rate. If it asks "between $x=2$ and $x=5$", it refers to the average rate.

---

Applications in Economics and Commerce

In the modern economic landscape, decision-making relies heavily on understanding how small changes in production affect financial outcomes. Calculus, specifically the derivative, allows economists to calculate marginal values. These values represent the instantaneous rate of change of a total quantity (like cost or revenue) with respect to the number of units produced or sold.

The Concept of Marginal Analysis

Marginal analysis is the study of the additional benefits or costs of an activity. If $x$ is the number of units of a commodity, then any function $f(x)$ representing a total quantity has a "marginal" counterpart $f'(x)$.

1. Marginal Cost (MC)

Marginal Cost is defined as the instantaneous rate of change of the Total Cost (C) with respect to the number of items produced ($x$). In simple terms, it is the approximate cost of producing one additional unit of the product.

Let $C(x)$ be the total cost function. The marginal cost $MC$ is given by the derivative:

Note: In many problems, $C(x)$ is expressed as a polynomial: $C(x) = ax^2 + bx + k$, where $k$ represents the Fixed Cost (cost when $x=0$) and $ax^2 + bx$ represents the Variable Cost.

2. Marginal Revenue (MR)

Marginal Revenue is the instantaneous rate of change of the Total Revenue (R) with respect to the number of items sold ($x$). It represents the additional income generated by selling one more unit of a product.

If $p$ is the price per unit (demand function) and $x$ is the number of units sold, then the Total Revenue $R(x)$ is calculated as:

The marginal revenue $MR$ is the derivative of this revenue function:

Related Economic Measures

To provide a comprehensive understanding for competitive exams, we must also consider Average Cost (AC) and Average Revenue (AR):

1. Average Cost (AC): It is the cost per unit of production.

2. Average Revenue (AR): It is the revenue per unit sold, which is identical to the price $p$.

---

Answer:

---

Answer:

---

Answer:

---

Answer:

---

Answer:

---

Answer:

---

Answer:

Tangents and Normals

The study of Tangents and Normals is one of the most direct geometric applications of differential calculus. By calculating the derivative of a function at a specific point, we gain precise information about the direction and orientation of the curve at that exact location. This is often referred to as finding the gradient of the curve.

---

Geometric Interpretation of the Derivative

Consider a continuous curve defined by the function $y = f(x)$. Let $P(x_1, y_1)$ be a fixed point on this curve. If we draw a straight line that just touches the curve at point $P$, this line is called the Tangent to the curve at $P$.

The Normal to a curve at a point $P(x_1, y_1)$ is a straight line that passes through $P$ and is perpendicular to the tangent at that same point.

Suppose the tangent line makes an angle $\psi$ with the positive direction of the $x$-axis. The slope ($m$) is defined as follows:

For the Normal line, using the condition of perpendicularity ($m_1 \cdot m_2 = -1$):

---

Special Geometric Cases

Case I: Tangent Parallel to the X-axis (Horizontal Tangent)

When the tangent is parallel to the $x$-axis, its inclination $\psi = 0^\circ$. This usually occurs at the turning points (maxima or minima) of a curve.

Case II: Tangent Parallel to the Y-axis (Vertical Tangent)

When the tangent is parallel to the $y$-axis, it is perpendicular to the $x$-axis. Here, $\psi = 90^\circ$. The slope becomes infinite, which mathematically implies the denominator of the derivative is zero.

Case III: Tangent Equally Inclined to the Axes

If a tangent is equally inclined to the coordinate axes, it makes an angle of $45^\circ$ or $135^\circ$ with the positive $x$-axis.

---

The Concept of the Normal

The Normal to a curve at a point $P(x_1, y_1)$ is defined as a straight line that passes through $P$ and is perpendicular to the tangent at that same point.

According to the laws of perpendicularity in coordinate geometry, if the slope of the tangent is $m_T$ and the slope of the normal is $m_N$, then their product must be $-1$.

By substituting the derivative for $m_T$, we derive the slope of the normal:

---

Summary Table for Quick Reference

Orientation of Tangent	Angle ($\psi$)	Mathematical Condition
Horizontal (Parallel to X-axis)	$0^\circ$	$\frac{dy}{dx} = 0$
Vertical (Parallel to Y-axis)	$90^\circ$	$\frac{dx}{dy} = 0$
Equally Inclined	$45^\circ / 135^\circ$	$\frac{dy}{dx} = \pm 1$
Perpendicular to a line with slope $m$	---	$\frac{dy}{dx} = -\frac{1}{m}$

---

Equations of Tangent and Normal

1. Equation of the Tangent

To find the equation of a tangent to the curve $y = f(x)$ at a given point $P(x_1, y_1)$, we utilize the Point-Slope Form of a straight line from coordinate geometry. This form states that a line passing through $(x_1, y_1)$ with a known slope $m$ is represented by the equation $y - y_1 = m(x - x_1)$.

In the context of calculus, the slope $m$ of the tangent at point $P$ is exactly the derivative of the function evaluated at that point.

Substituting this value into the point-slope formula, we obtain the Equation of the Tangent:

Special Remark

If the derivative $\frac{dy}{dx}$ does not exist (becomes infinite) at the point $(x_1, y_1)$, the tangent is vertical (parallel to the $y$-axis). In such a case, the equation of the tangent simplifies to:

2. Equation of the Normal

The Normal to a curve at a point $P(x_1, y_1)$ is defined as the line perpendicular to the tangent at that point. From the properties of perpendicular lines in coordinate geometry, we know that the product of the slopes of two perpendicular lines is $-1$.

Thus, the slope of the normal ($m_n$) can be derived as:

By applying the point-slope form again, we find the Equation of the Normal at $(x_1, y_1)$:

This equation can also be written in an alternative linear form to avoid fractions:

---

Angle of Intersection of Two Curves

The angle of intersection between two curves is a fundamental concept in coordinate geometry and calculus. It is defined as the angle between the tangents drawn to the two curves at their common point of intersection. If the two tangents are perpendicular, the curves are said to intersect orthogonally.

Geometric Representation

Let two curves be represented by the functions $y = f(x)$ and $y = g(x)$. Suppose these curves intersect at a point $P(x_1, y_1)$. At this point, we can draw a tangent $T_1$ to the first curve and a tangent $T_2$ to the second curve. The angle $\theta$ between these two tangents is the angle of intersection of the curves.

Formula and Derivation

Let $m_1$ be the slope of the tangent to the curve $y = f(x)$ at point $P$, and $m_2$ be the slope of the tangent to the curve $y = g(x)$ at point $P$.

From trigonometry, the acute angle $\theta$ between two lines with slopes $m_1$ and $m_2$ is given by:

Special Conditions for Intersection

1. Orthogonal Intersection

Two curves are said to cut each other orthogonally (at right angles) if the tangents at their point of intersection are perpendicular to each other. In this case, $\theta = 90^\circ$.

2. Tangential Curves (Touching Curves)

Two curves are said to touch each other if the tangents at their point of intersection coincide. In this case, the angle $\theta = 0^\circ$.

Step-by-Step Procedure to find Angle of Intersection

To find the angle between two curves, follow these steps:

Step I: Solve the equations of the two given curves simultaneously to find the Point of Intersection $(x_1, y_1)$. If there are multiple points, calculate the angle at each point separately.

Step II: Find the derivative $\frac{dy}{dx}$ for both curves separately.

Step III: Calculate the numerical values of the slopes $m_1$ and $m_2$ by substituting the coordinates of the intersection point into the respective derivatives.

Step IV: Substitute $m_1$ and $m_2$ into the $\tan \theta$ formula to find the acute angle.

---

Answer:

---

Answer:

Approximations, Errors and Differentials

The concept of Approximation using derivatives is based on the idea of linearization. When we look at a curve very closely at a specific point, it behaves almost like a straight line (the tangent). Differentials allow us to use this straight-line behavior to estimate changes in a function without performing complex calculations.

---

Conceptual Definition of Differentials

Let $y = f(x)$ be a differentiable function. Suppose $x$ is an independent variable that undergoes a small change, denoted by $\delta x$. This change in $x$ causes a corresponding change in the dependent variable $y$, which we denote as $\delta y$.

The Mathematical Derivation

By the first principle of derivatives, the derivative of $y$ with respect to $x$ is defined as the limit of the ratio of these changes:

For a non-zero but very small increment $\delta x$, the ratio $\frac{\delta y}{\delta x}$ is approximately equal to the derivative $\frac{dy}{dx}$. We can express this relationship by introducing a small quantity $\epsilon$ that vanishes as $\delta x$ approaches zero:

Multiplying both sides by $\delta x$:

Since $\epsilon \cdot \delta x$ is the product of two extremely small quantities, it becomes negligible. Therefore, for practical calculation and approximation, we have:

---

Defining the Differentials $dx$ and $dy$

To make the notation consistent with the symbol $\frac{dy}{dx}$, we formally define the differentials as separate mathematical entities:

A. Differential of the Independent Variable ($dx$)

The differential of the independent variable $x$, denoted by $dx$, is defined to be exactly equal to its increment $\delta x$. It represents an arbitrary change in $x$.

B. Differential of the Dependent Variable ($dy$)

The differential of the dependent variable $y$, denoted by $dy$, is defined as the product of the derivative $f'(x)$ and the differential $dx$.

---

Critical Distinction: $\delta y$ vs $dy$

It is crucial for students to understand that $\delta y$ and $dy$ are not the same, although they are very close in value when $\delta x$ is small.

1. Actual Change ($\delta y$): This is the true difference in the function's value, $f(x + \delta x) - f(x)$. Geometrically, it represents the change in height along the curve.

2. Differential Change ($dy$): This is the principal part of the change. Geometrically, it represents the change in height along the tangent line to the curve at point $x$.

In most approximation problems, we treat $dy$ as a reliable substitute for $\delta y$. This leads to the general formula for approximating a value near a known point:

---

Detailed Application to Approximations

To find the approximate value of a function $f$ at a point $x + \delta x$, where $\delta x$ is very small, we use the concept of linear approximation. The underlying principle is that for a very small interval, the curve of a function is nearly identical to its tangent line.

Derivation of the Approximation Formula

Let $y = f(x)$. If $x$ increases to $x + \delta x$, then the actual change in $y$ is denoted by $\delta y$. We can write this relationship as:

By subtracting $y = f(x)$ from both sides, we isolate the change in $y$:

We know from the definition of differentials that the actual change $\delta y$ is approximately equal to the differential $dy$. Therefore:

Substituting the expression for $dy = f'(x) \delta x$ into the above equation:

Rearranging the formula gives us the final tool for calculation:

Working Strategy: To use this formula, we split the given number into two parts: $x$ (a value whose function result is perfectly known) and $\delta x$ (the small remaining part or error).

---

Classification of Errors

In scientific measurements and engineering calculations, no measurement is perfectly precise. The difference between the measured value and the actual value is termed as an "error". Calculus helps us determine how an error in an independent variable propagates to the dependent variable.

Definitions of Error Types

Suppose $x$ is the measured value of a quantity and $\delta x$ is the error in its measurement:

Type of Error	Mathematical Formula	Description
Absolute Error	$\delta x$	The actual magnitude of the error in measurement of $x$.
Relative Error	$\frac{\delta x}{x}$	The ratio of the absolute error to the true value of the quantity.
Percentage Error	$\frac{\delta x}{x} \times 100$	The relative error expressed as a part of 100.

Propagated Error in Dependent Variables

If $y = f(x)$ is a calculated quantity based on the measured value $x$, the absolute error in $y$ ($\delta y$) can be estimated using the derivative:

Similarly, the Relative Error in $y$ is given by $\frac{\delta y}{y}$ and the Percentage Error in $y$ is $\frac{\delta y}{y} \times 100$.

---

Geometrical Interpretation of Differentials

Consider the graph of a differentiable function $y = f(x)$. Let $P(x, y)$ be a point on the curve. Suppose the independent variable $x$ increases by a small amount $\delta x$ to reach a new value $x + \delta x$. This corresponds to a new point $Q(x + \delta x, y + \delta y)$ on the curve.

The Construction

1. Draw a tangent to the curve at point $P$. Let this tangent make an angle $\psi$ with the positive $x$-axis.

2. From point $Q$, drop a perpendicular to the $x$-axis. Let it intersect the horizontal line through $P$ at point $M$ and the tangent line at point $R$.

3. Here, the distance $PM$ represents the change in $x$, so $PM = \delta x = dx$.

Formal Derivation of the Relationship

From the right-angled triangle $\triangle PRM$ in the figure above, we can define the slope of the tangent line:

We know from the geometric definition of the derivative that the slope of the tangent at $P$ is $\frac{dy}{dx}$. Also, since $PM = dx$:

By cross-multiplying, we find the value of the vertical segment $RM$:

By our mathematical definition, $dy = f'(x) dx$. Therefore, we can conclude that:

Distinguishing the Changes

Now, let us observe the segment $QM$ in the diagram:

1. Actual Increment ($\delta y$): The segment $QM$ represents the true change in the function's value as we move along the curve from $P$ to $Q$. Thus, $QM = \delta y$.

2. Differential ($dy$): The segment $RM$ represents the change in the vertical coordinate along the tangent line. Thus, $RM = dy$.

3. The Approximation Error: The small segment $QR$ represents the difference between the actual change and the differential. Mathematically, $\text{Error} = \delta y - dy$.

The Principle of Linearization

As the change in $x$ becomes smaller and smaller ($\delta x \to 0$), the point $Q$ moves closer to $P$ along the curve. The segment $QR$ shrinks much faster than the segments $QM$ or $RM$. Consequently, the tangent line becomes an almost perfect substitute for the curve in the immediate neighborhood of $P$.

This is why we can confidently state:

---

Answer:

---

Answer:

Increasing and Decreasing Functions

The concept of Monotonicity refers to the behavior of a function—whether it moves in an upward (increasing) or downward (decreasing) direction as the input value $x$ increases. In calculus, this behavior is determined by the rate of change of the function, which is represented by its derivative $f'(x)$.

If we consider a function $y = f(x)$, the derivative $f'(x)$ at any point represents the slope of the tangent to the curve at that point. The sign of this slope dictates the nature of the function:

The Mathematical Relationship

To determine where a function is increasing or decreasing, we must analyze the intervals where the derivative maintains a constant sign. This is achieved by solving polynomial or rational inequalities. The process generally involves the following steps:

1. Finding Critical Points: We find the values of $x$ where $f'(x) = 0$ or where $f'(x)$ is not defined. These points are where the function might change its behavior from increasing to decreasing or vice versa.

2. Dividing the Domain: These critical points divide the domain of the function into several disjoint open intervals.

3. Testing Signs: We check the sign of $f'(x)$ in each interval. If the derivative is positive throughout an interval, the function is strictly increasing there. If it is negative, the function is strictly decreasing.

Importance of Inequality Solving

Since the core of this section involves solving $f'(x) > 0$ and $f'(x) < 0$, mastering the Method of Intervals (also known as the Wavy Curve Method) is a prerequisite. Without a systematic way to solve these inequalities, it is impossible to accurately identify the intervals of monotonicity for complex functions like:

$f'(x) = \frac{(x-1)^3(x+2)}{(x-4)^2}$

Summary Table of Monotonicity

Condition of Derivative	Nature of Function $f(x)$
$f'(x) > 0$ for all $x \in (a, b)$	Strictly Increasing
$f'(x) \geq 0$ for all $x \in (a, b)$	Increasing (Non-decreasing)
$f'(x) < 0$ for all $x \in (a, b)$	Strictly Decreasing
$f'(x) \leq 0$ for all $x \in (a, b)$	Decreasing (Non-increasing)
$f'(x) = 0$ for all $x \in (a, b)$	Constant Function

---

Method of Intervals (Wavy Curve Method)

The Method of Intervals, popularly known as the Wavy Curve Method, is a powerful strategy used to solve inequalities of the form $f(x) > 0, f(x) < 0, f(x) \geq 0$ or $f(x) \leq 0$, where $f(x)$ is a polynomial or a rational function. This method helps in finding the intervals where the function remains positive or negative without testing every single point.

Consider a general polynomial expression $f(x)$ which has been factorised into linear factors:

Where $a_1, a_2, \dots, a_n$ are distinct real roots (critical points) and $k_1, k_2, \dots, k_n$ are natural numbers representing the multiplicity (powers) of these roots. We assume the roots are arranged such that:

$a_1 < a_2 < a_3 < \dots < a_n$

Sign Determination Rules and Logic

The sign of the expression changes or stays the same based on the power of the factor as we cross a critical point on the number line.

Rule 1: Even Powers (Multiplicity is Even)

If a factor is raised to an even power, such as $(x - a)^{2}, (x - a)^{4}$, etc., the term $(x - a)^k$ is always non-negative for all real $x$.

Since a positive number does not change the sign of the product, the sign of $f(x)$ does not change as $x$ moves from one side of $a$ to the other. For the purpose of the wavy curve, we can effectively "ignore" such factors, but we must account for them if the inequality is $\geq 0$ or $\leq 0$ (as the expression becomes zero at $x = a$).

Rule 2: Odd Powers (Multiplicity is Odd)

If a factor is raised to an odd power, such as $(x - a)^{1}, (x - a)^{3}$, etc., the sign of $(x - a)^k$ is exactly the same as the sign of $(x - a)$.

As $x$ crosses the root $a$, the factor $(x - a)$ changes from negative to positive. Consequently, the sign of the entire expression $f(x)$ changes (flips).

Step-by-Step Procedure for Wavy Curve

To implement this method efficiently, follow these structural steps:

Step 1: Standardise the Factors

Ensure that the coefficient of $x$ in every linear factor is positive (preferably $+1$). If a factor is $(a - x)$, rewrite it as $-(x - a)$ and adjust the inequality sign if necessary (multiplying by $-1$ reverses the inequality).

Step 2: Plot Critical Points

Identify all roots by setting each factor to zero. Mark these points $a_1, a_2, \dots, a_n$ on a horizontal real number line.

Step 3: Assign Signs to Intervals

1. Rightmost Interval: For the interval to the extreme right (where $x > a_n$), the expression $f(x)$ will always be positive (+), provided all factors were standardised to the $(x - a)$ form. This is because every single factor in the product will be positive.

2. Moving Leftwards: As you move to the next interval by crossing a root:

$\bullet$ If the root comes from a factor with an odd power, change the sign (from $+$ to $-$ or vice versa).

$\bullet$ If the root comes from a factor with an even power, maintain the same sign.

Step 4: Identify the Solution Set

Based on the original inequality:

$\bullet$ If $f(x) > 0$, choose all intervals marked with a (+) sign.

$\bullet$ If $f(x) < 0$, choose all intervals marked with a (–) sign.

$\bullet$ For $\geq$ or $\leq$, include the roots themselves (closed brackets), but exclude any roots that make the denominator zero in the case of rational functions.

---

Rational Inequalities

A Rational Inequality involves a ratio of two polynomial functions, generally expressed in the form $Q(x) = \frac{f(x)}{g(x)}$. The fundamental objective is to determine the range of values for $x$ such that the quotient is either positive, negative, or zero. Solving these is very similar to solving polynomial inequalities, but with specific constraints regarding the denominator.

Let the rational expression be defined as:

Where $a_i$ are the roots of the numerator and $b_j$ are the roots of the denominator.

The Principle of Sign Equivalence

The core logic behind solving rational inequalities is that the sign (positive or negative) of a quotient $\frac{a}{b}$ is identical to the sign of the product $a \cdot b$. This is because in both multiplication and division, the result is positive if the signs are the same and negative if the signs are opposite.

Mathematical Derivation

To convert a rational inequality into a polynomial-like form without changing the sense of the inequality, we can multiply both sides by the square of the denominator. Since the square of any real non-zero number is always positive, the inequality remains valid.

Consider the inequality:

Multiply both sides by $[g(x)]^2$ (given $g(x) \neq 0$):

Simplifying the left-hand side:

Thus, the sign of the rational function $\frac{f(x)}{g(x)}$ is exactly the same as the sign of the product $f(x) \cdot g(x)$. This allows us to use the Wavy Curve Method on the combined roots of both the numerator and the denominator.

Critical Constraints and Domain

While the sign determination is similar to polynomials, we must strictly adhere to the following rules regarding the denominator:

1. Undefined Points: The denominator $g(x)$ can never be zero. Therefore, even if the inequality is non-strict ($\geq$ or $\leq$), the roots of $g(x)$ must always be excluded from the solution set using open intervals or parenthesis.

2. Common Factors: If there is a common factor $(x - c)$ in both the numerator and denominator, it cancels out for sign determination, but the point $x = c$ must still be excluded from the domain as the original function remains undefined at that point.

Comparison of Conditions

Quotient Form	Equivalent Product Form	Constraint
$\frac{f(x)}{g(x)} > 0$	$f(x) \cdot g(x) > 0$	$g(x) \neq 0$
$\frac{f(x)}{g(x)} \geq 0$	$f(x) \cdot g(x) \geq 0$	$g(x) \neq 0$ (Exclude $b_j$ roots)
$\frac{f(x)}{g(x)} < 0$	$f(x) \cdot g(x) < 0$	$g(x) \neq 0$

---

Answer:

---

Answer:

---

Answer:

---

Increasing and Decreasing Functions

Let $f$ be a real-valued function defined in a domain $D$, where $D$ is a subset of the set of real numbers $\mathbb{R}$. We analyze the behavior of the function over an interval $D_1$ (which is a subset of $D$) based on how the output values $f(x)$ change relative to the input values $x$.

Increasing and Strictly Increasing Functions

In mathematical analysis, the behavior of a real-valued function $f(x)$ as the input $x$ moves from left to right on the real number line is referred to as its Monotonicity. When a function consistently maintains an upward trend or does not decrease, we classify it as an Increasing Function or a Strictly Increasing Function.

Let $f$ be a real-valued function defined on an interval $D \subseteq \mathbb{R}$, and let $D_1$ be a subset of $D$.

1. Increasing Function (Non-decreasing)

A function $f$ is said to be an increasing function on an interval $D_1$ if for any two points $x_1$ and $x_2$ in that interval, the order of the function values follows the order of the inputs, allowing for the possibility that the values may remain equal over some sub-interval.

Mathematically, the condition is defined as:

Graphical Interpretation: The graph of an increasing function never moves downwards. It may move upwards or it may stay horizontal (flat) for some time. In terms of calculus, the slope of the tangent to such a curve is never negative, meaning the derivative satisfies:

2. Strictly Increasing Function

A function $f$ is strictly increasing on $D_1$ if a greater input always results in a strictly greater output. There are no "flat" or constant portions in the graph of a strictly increasing function.

The rigorous definition is:

Graphical Interpretation: The graph always moves upwards as we move from left to right. There are no horizontal segments. For a differentiable function, this usually implies that the derivative is positive:

---

Key Differences and Summary

The fundamental difference lies in the strictness of the inequality. While every strictly increasing function is technically also an increasing function, the reverse is not always true.

Feature	Increasing Function	Strictly Increasing Function
Condition	$f(x_1) \leq f(x_2)$	$f(x_1) < f(x_2)$
Horizontal Segments	Permitted (Graph can stay flat)	Not Permitted (Graph must rise)
Derivative ($f'$)	$f'(x) \geq 0$	$f'(x) > 0$
Nature	Non-decreasing	Always increasing

---

Answer:

---

Decreasing and Strictly Decreasing Functions

In the study of monotonocity, when a function consistently maintains a downward trend or does not increase as the input $x$ grows, it is classified under Decreasing or Strictly Decreasing functions. This behavior indicates that the output $f(x)$ is inversely related to the input $x$.

Let $f$ be a real-valued function defined on an interval $D \subseteq \mathbb{R}$, and let $D_2$ be a sub-interval of $D$.

3. Decreasing Function (Non-increasing)

A function $f$ is termed a decreasing function on an interval $D_2$ if, for any two values in that interval, the larger input results in an output that is either smaller than or equal to the output of the smaller input. This allows for portions of the graph to be horizontal.

The formal mathematical definition is:

Graphical Interpretation: The graph of a decreasing function never rises as it moves from left to right. It may descend or remain flat (constant). In the context of calculus, the slope of the tangent (derivative) at any point is either negative or zero:

4. Strictly Decreasing Function

A function $f$ is strictly decreasing on $D_2$ if every increase in the input $x$ results in a definite decrease in the output $f(x)$. Unlike a standard decreasing function, there are no constant or horizontal segments in its graph.

The formal mathematical definition is:

Graphical Interpretation: The graph strictly falls as we move along the positive x-axis. For differentiable functions, the derivative is strictly negative:

---

Comparison Table of Decreasing Nature

Function Type	Inequality Condition	Derivative Property	Graph Behavior
Decreasing	$f(x_1) \geq f(x_2)$	$f'(x) \leq 0$	Falls or stays flat
Strictly Decreasing	$f(x_1) > f(x_2)$	$f'(x) < 0$	Always falls

---

Answer:

---

Monotonic Functions

In mathematical analysis, the term Monotonic is derived from the Greek words 'monos' (single) and 'tonos' (tone or direction). A function is considered monotonic over an interval if it preserves a consistent direction of movement—it either never decreases or never increases as the input $x$ moves across the interval.

Monotonicity is a fundamental property used to analyze the global and local behavior of functions, particularly in solving complex inequalities and optimization problems.

Classification of Monotonicity

A function $f$ defined on an interval $D$ is classified based on the strictness of its directional consistency:

1. Monotonic Function

A function is called monotonic in an interval if it is either increasing or decreasing throughout that interval. In this case, the function is allowed to remain constant (horizontal) over some parts of the interval.

2. Strictly Monotonic Function

A function is called strictly monotonic in an interval if it is either strictly increasing or strictly decreasing throughout that interval. Such functions never stay constant; they are always either rising or falling.

Summary of Monotonicity Conditions

The following table summarizes the relationship between the input values, the function outputs, and the visual behavior of the graph:

Nature of Function	Condition for $x_1 < x_2$	Graphical Interpretation
Increasing (Non-decreasing)	$f(x_1) \leq f(x_2)$	The graph moves upward or remains horizontal.
Strictly Increasing	$f(x_1) < f(x_2)$	The graph moves upward only (no flat regions).
Decreasing (Non-increasing)	$f(x_1) \geq f(x_2)$	The graph moves downward or remains horizontal.
Strictly Decreasing	$f(x_1) > f(x_2)$	The graph moves downward only (no flat regions).

Mathematical Significance of Monotonicity

1. Inverse Functions: A function has an inverse (is invertible) if and only if it is strictly monotonic over its entire domain. This ensures that the function is "One-to-One" (Injective).

2. Derivative Relationship: For a differentiable function $f(x)$, monotonicity is determined by the sign of the first derivative $f'(x)$.

---

Conditions for an Increasing or a Decreasing Function

The derivative of a function $f(x)$ serves as a mathematical tool to determine the intervals of monotonicity. Since the derivative $f'(x)$ represents the slope of the tangent at any point $P(x, y)$ on the curve, its sign indicates whether the function is ascending or descending.

Geometrical Interpretation

The derivative of a function at any given point provides the slope of the tangent to the curve at that point. By analyzing the angle that this tangent makes with the positive direction of the x-axis, we can geometrically determine whether a function is increasing or decreasing.

1. Strictly Increasing Behavior and Acute Angles

For a function $f(x)$ that is strictly increasing in an interval, the curve consistently rises as we move from left to right. Consequently, the tangent drawn at any point on such a curve will lean towards the right.

Let $\psi$ be the angle formed by the tangent with the positive direction of the x-axis. For a strictly increasing function, this angle $\psi$ is typically an acute angle (i.e., $0 < \psi < 90^\circ$).

From the principles of trigonometry, we know that the tangent of an acute angle is always positive:

Since the derivative $f'(x)$ is equal to $\tan \psi$, we conclude:

2. Strictly Decreasing Behavior and Obtuse Angles

Conversely, if a function $f(x)$ is strictly decreasing, the curve falls as $x$ increases. The tangent drawn at any point on this curve leans towards the left, forming an obtuse angle ($\psi$) with the positive direction of the x-axis (i.e., $90^\circ < \psi < 180^\circ$).

The tangent of an obtuse angle (in the second quadrant) is always negative:

This implies that the derivative must be negative for the function to be strictly decreasing:

The Nuanced Case: Analyzing $f(x) = x^3$

While the sign of $f'(x)$ is a strong indicator, geometric intuition can be refined by observing functions where the derivative becomes zero at specific points without the function losing its strictly increasing nature.

Consider the cubic function $f(x) = x^3$. Let us analyze its behavior across the entire set of real numbers $\mathbb{R}$. For any two values $x_1, x_2 \in \mathbb{R}$:

This confirms that $f(x) = x^3$ is strictly increasing on $\mathbb{R}$. However, let us look at its derivative:

At the origin ($x = 0$):

Geometrically, this means that at $x = 0$, the tangent is horizontal (parallel to the x-axis). Although the slope is not strictly positive at this single isolated point, the function does not "flatten out" over an interval. It merely has a point of inflection where the rate of change momentarily becomes zero before continuing its upward journey.

Conclusion on Geometrical Conditions

Based on these observations, we refine our criteria. A function is strictly increasing if its tangent slope is generally positive. Even if the slope becomes zero at finite isolated points, the function remains strictly monotonic. However, if the slope remains zero over a continuous interval, the function becomes a constant function over that interval and is no longer "strictly" increasing.

---

Fundamental Theorems on Monotonicity

The relationship between the derivative of a function and its monotonic behavior is formalized through two key theorems. These theorems allow us to use the tools of calculus to determine whether a function is increasing or decreasing over a specific interval without testing every individual point.

For these theorems, we assume that the function $f$ satisfies two primary conditions:

1. $f$ is continuous in the closed interval $[a, b]$.

2. $f$ is derivable (differentiable) in the open interval $(a, b)$.

Theorem 1: Conditions for Increasing Nature

This theorem provides the criteria for identifying non-decreasing and strictly rising behavior of a function.

(i) Condition for Increasing (Non-decreasing) Function

If $f'(x) \geq 0$ for all $x \in (a, b)$, then $f$ is increasing in $[a, b]$. In this scenario, the function's value either rises or stays constant, but never falls.

(ii) Condition for Strictly Increasing Function

If $f'(x) > 0$ for all $x \in (a, b)$, then $f$ is strictly increasing in $[a, b]$. This implies that for every increase in $x$, there is a guaranteed increase in $f(x)$.

Theorem 2: Conditions for Decreasing Nature

Similarly, these conditions define when a function moves downward or stays non-rising.

(i) Condition for Decreasing (Non-increasing) Function

If $f'(x) \leq 0$ for all $x \in (a, b)$, then $f$ is decreasing in $[a, b]$. The function value either falls or remains horizontal.

(ii) Condition for Strictly Decreasing Function

If $f'(x) < 0$ for all $x \in (a, b)$, then $f$ is strictly decreasing in $[a, b]$. The function value consistently reduces as $x$ increases.

Derivation of the Theorems

The formal proof of these theorems is rooted in Lagrange’s Mean Value Theorem (LMVT). Let us derive the condition for a strictly increasing function.

To Prove: If $f'(x) > 0$ for all $x \in (a, b)$, then $f(x)$ is strictly increasing.

Proof:

Let $x_1$ and $x_2$ be any two points in the interval $[a, b]$ such that $x_1 < x_2$. Since $f(x)$ is continuous on $[x_1, x_2]$ and differentiable on $(x_1, x_2)$, by applying Lagrange’s Mean Value Theorem, there exists at least one point $c \in (x_1, x_2)$ such that:

This can be rearranged as:

Now, let us analyze the signs of the terms on the right-hand side:

If $f'(x) > 0$ for all $x$ in the interval, then $f'(c) > 0$. Multiplying two positive quantities in equation (ii) gives:

Since $x_1 < x_2 \implies f(x_1) < f(x_2)$, the function is strictly increasing. The same logic applies to decreasing functions by substituting $f'(c) < 0$.

---

The Discrete Zero Property

The Discrete Zero Property is a vital extension of the first derivative test. In many real-world and mathematical functions, the derivative might become zero at certain specific points without the function losing its "strictly" monotonic nature.

A set of points is called discrete or isolated if each point has a neighborhood that contains no other points from the set. In simpler terms, the points where the derivative is zero are "scattered" and do not form a continuous line segment.

Formal Statement of the Corollary

Let $f(x)$ be a function that is continuous on the closed interval $[a, b]$ and differentiable on the open interval $(a, b)$.

1. For Strictly Increasing Nature

If $f'(x) \geq 0$ for all $x \in (a, b)$ and $f'(x) = 0$ only at a finite number of isolated points, then $f(x)$ is strictly increasing on $[a, b]$.

2. For Strictly Decreasing Nature

If $f'(x) \leq 0$ for all $x \in (a, b)$ and $f'(x) = 0$ only at a finite number of isolated points, then $f(x)$ is strictly decreasing on $[a, b]$.

Why does this property hold?

The core logic lies in the definition of strictly increasing behavior: $x_1 < x_2 \implies f(x_1) < f(x_2)$. If the derivative is zero only at an isolated point (like a "momentary pause"), the function immediately continues its upward or downward journey after that point. Since the function does not stay constant over any interval, the output value $f(x)$ will always be greater for a larger $x$.

Mathematical Comparison

Derivative Behavior	Monotonic Nature	Example
$f'(x) > 0$ always	Strictly Increasing	$e^x$
$f'(x) \geq 0$ and $f'(x) = 0$ at isolated points	Strictly Increasing	$x^3$
$f'(x) = 0$ over a sub-interval $(c, d)$	Increasing (but not strictly)	Step functions

---

Answer:

---

Geometrical Concept: Point of Inflection

In cases where the derivative is zero at a discrete point but the function remains strictly monotonic, that point is often a point of inflection. At such a point, the tangent is horizontal, but the curve does not turn back; it continues its original trend. This is visually represented in the graph of $y = x^3$ at the origin.

---

Summary of Derivative Tests for Monotonicity

The First Derivative Test is the most efficient method to determine the monotonic behavior of a differentiable function. By analyzing the sign of $f'(x)$ over a specific interval, we can conclude whether the function is rising, falling, or remaining constant.

Comprehensive Derivative Test Table

The following table summarizes the relationship between the sign of the first derivative and the nature of the function $f(x)$ in an interval $(a, b)$ where $f(x)$ is continuous and differentiable.

Sign of $f'(x)$	Monotonic Nature of $f(x)$
$f'(x) > 0$	Strictly Increasing
$f'(x) \geq 0$	Increasing (Non-decreasing)
$f'(x) < 0$	Strictly Decreasing
$f'(x) \leq 0$	Decreasing (Non-increasing)
$f'(x) = 0$	Constant Function

Detailed Analysis of Conditions

1. Strictly Increasing Functions ($f'(x) > 0$)

When the derivative is strictly positive for all $x$ in an interval, the slope of the tangent is always positive. This means the function value $f(x)$ always increases as $x$ increases. There are no points where the graph "levels off" or stays horizontal.

2. Increasing/Non-decreasing Functions ($f'(x) \geq 0$)

If the derivative is greater than or equal to zero, the function never decreases. It may rise ($f'(x) > 0$) or it may remain constant ($f'(x) = 0$) over a sub-interval. This is a broader category that includes strictly increasing functions.

3. Strictly Decreasing Functions ($f'(x) < 0$)

A strictly negative derivative indicates that the tangent at every point makes an obtuse angle with the positive x-axis. As $x$ increases, $f(x)$ consistently decreases without any horizontal pauses.

4. Decreasing/Non-increasing Functions ($f'(x) \leq 0$)

If the derivative is less than or equal to zero, the function never rises. It can either fall or remain at a constant value for some duration. This category includes strictly decreasing functions.

5. Constant Functions ($f'(x) = 0$)

If the derivative is zero throughout an entire interval, the rate of change is zero. The function does not increase or decrease, resulting in a horizontal straight line.

Special Condition: The Discrete Zero Case

A critical refinement of the "Strictly Increasing" condition is used. If the derivative $f'(x)$ is zero only at isolated points (discrete points) and positive elsewhere, the function remains strictly increasing.

This allows us to classify functions like $f(x) = x^3$ as strictly increasing even though $f'(0) = 0$. Because the zero is momentary and doesn't occur over an interval, the strict inequality $f(x_1) < f(x_2)$ is never violated.

---

Answer:

---

Answer:

Maxima and Minima

Finding the extreme values of a function, i.e., its highest and lowest points, is a fundamental problem in calculus and has numerous applications in optimisation. Derivatives play a crucial role in locating these values. The extreme values can be classified as absolute (global) or local (relative).

---

Absolute Maximum and Minimum Values (Global Extrema)

The absolute maximum or minimum value of a function over a given interval is the largest or smallest value that the function attains anywhere in that entire interval.

Let $f$ be a function defined on an interval $I$.

• A function $f$ is said to have an absolute maximum value on $I$ at a point $c \in I$ if $f(c) \geq f(x)$ for all $x$ in the interval $I$. The value $f(c)$ is the absolute maximum value of $f$ on $I$, and $c$ is the point of absolute maximum.
• A function $f$ is said to have an absolute minimum value on $I$ at a point $c \in I$ if $f(c) \leq f(x)$ for all $x$ in the interval $I$. The value $f(c)$ is the absolute minimum value of $f$ on $I$, and $c$ is the point of absolute minimum.

The absolute maximum and minimum values are collectively called the **global extrema** or **absolute extrema** of the function on the interval $I$. A function may or may not have absolute extrema on an open interval or the entire real line. However, a key theorem guarantees their existence under certain conditions:

Extreme Value Theorem:

If a function $f$ is continuous on a closed interval $[a, b]$, then $f$ is guaranteed to attain both an absolute maximum value and an absolute minimum value at some points within that interval $[a, b]$.

For a continuous function on a closed interval, these absolute extreme values must occur either at the endpoints of the interval ($x=a$ or $x=b$) or at the critical points of the function within the open interval $(a, b)$.

Critical Point

A critical point (or critical number) of a function $f$ is a point $c$ in the domain of $f$ where either:

• $f'(c) = 0$ (the tangent line is horizontal), OR
• $f'(c)$ is undefined.

Critical points are candidates for local (and sometimes global) maxima or minima because the function's behaviour (increasing/decreasing) can change at these points.

Procedure for Finding Absolute Maximum and Minimum Values on a Closed Interval $[a, b]$

If a function $f$ is continuous on a closed interval $[a, b]$, the following systematic approach can be used to find its absolute maximum and minimum values:

1. Find all critical points of the function $f(x)$ that lie within the open interval $(a, b)$. To do this, compute the derivative $f'(x)$, set $f'(x) = 0$ and solve for $x$, and also identify any points in $(a, b)$ where $f'(x)$ is undefined but $f(x)$ is defined.

2. Evaluate the function $f(x)$ at all the critical points found in step 1.

3. Evaluate the function $f(x)$ at the endpoints of the interval, i.e., find $f(a)$ and $f(b)$.

4. Compare all the function values calculated in steps 2 and 3.

• The largest among these values is the absolute maximum value of $f$ on $[a, b]$.
• The smallest among these values is the absolute minimum value of $f$ on $[a, b]$.

The point(s) where the absolute maximum value occurs is/are the point(s) of absolute maximum, and similarly for the absolute minimum.

---

Answer:

---

Answer:

Local Maximum and Local Minimum Values

Beyond finding the absolute highest and lowest points on an interval, derivatives also help us identify local (or relative) extrema. These are points where the function reaches a peak or a valley in its immediate neighbourhood, even if there are higher or lower points elsewhere in the domain.

---

Definitions

Let $f$ be a function defined on an interval $I$, and let $c$ be an interior point of $I$ (a point that is not an endpoint).

• A function $f$ is said to have a local maximum value at $c$ if there exists at least one open interval $(c - \delta, c + \delta)$, where $\delta > 0$ is a small positive number, such that $(c - \delta, c + \delta) \subset I$ and $f(c) \geq f(x)$ for all $x$ in $(c - \delta, c + \delta)$. The value $f(c)$ is called a local maximum value of $f$. This means $f(c)$ is the highest function value in some neighbourhood around $c$.
• A function $f$ is said to have a local minimum value at $c$ if there exists at least one open interval $(c - \delta, c + \delta)$, where $\delta > 0$, such that $(c - \delta, c + \delta) \subset I$ and $f(c) \leq f(x)$ for all $x$ in $(c - \delta, c + \delta)$. The value $f(c)$ is called a local minimum value of $f$. This means $f(c)$ is the lowest function value in some neighbourhood around $c$.

Points where a function has a local maximum or a local minimum are collectively called local extrema or relative extrema. A point $c$ where a local extremum occurs must be a critical point of the function (where $f'(c) = 0$ or $f'(c)$ is undefined, provided $c$ is in the domain). This is a necessary condition, but not sufficient (i.e., not all critical points are local extrema).

---

Tests for Local Extrema

To determine whether a critical point corresponds to a local maximum, a local minimum, or neither, we use tests based on the derivative(s) of the function.

First Derivative Test

The First Derivative Test relies on examining the sign of the first derivative, $f'(x)$, as $x$ moves across a critical point. The sign of the derivative tells us whether the function is increasing or decreasing in the vicinity of the critical point.

Let $c$ be a critical point of a continuous function $f$ defined on an interval $I$. Suppose $f$ is differentiable in $(c-\delta, c)$ and $(c, c+\delta)$ for some $\delta > 0$.

• Condition for Local Maximum: If $f'(x)$ changes sign from positive to negative as $x$ increases through $c$, then $f$ has a local maximum at $c$. Graphically, the function increases up to $c$ and then decreases after $c$, forming a peak.

Sign of $f'(x)$ for $x < c$: Positive ($f$ increasing)

Sign of $f'(x)$ for $x > c$: Negative ($f$ decreasing)

$\implies$ Local Maximum at $c$.

• Condition for Local Minimum: If $f'(x)$ changes sign from negative to positive as $x$ increases through $c$, then $f$ has a local minimum at $c$. Graphically, the function decreases up to $c$ and then increases after $c$, forming a valley.

Sign of $f'(x)$ for $x < c$: Negative ($f$ decreasing)

Sign of $f'(x)$ for $x > c$: Positive ($f$ increasing)

$\implies$ Local Minimum at $c$.

• Condition for Neither Maximum nor Minimum: If $f'(x)$ does not change sign as $x$ increases through $c$ (i.e., $f'(x)$ is positive on both sides of $c$ or negative on both sides), then $c$ is neither a local maximum nor a local minimum. In such cases, the function continues to increase or decrease through the critical point. (Note: While $f'(c)=0$ or is undefined, $c$ might be a point of inflection, related to concavity change, but the First Derivative Test only tells us about monotonicity change).

Sign of $f'(x)$ for $x < c$: Positive

Sign of $f'(x)$ for $x > c$: Positive

$\implies$ Neither Local Max nor Min at $c$ (possibly point of inflection).

Sign of $f'(x)$ for $x < c$: Negative

Sign of $f'(x)$ for $x > c$: Negative

$\implies$ Neither Local Max nor Min at $c$ (possibly point of inflection).

Procedure using First Derivative Test:

1. Find the derivative $f'(x)$.

2. Find the critical points of $f(x)$ (where $f'(x)=0$ or $f'(x)$ is undefined) that are in the domain of $f$.

3. Arrange the critical points in increasing order on a number line. These points divide the domain into disjoint intervals.

4. For each critical point $c$, examine the sign of $f'(x)$ in an open interval immediately to the left of $c$ and in an open interval immediately to the right of $c$. This can be done by picking a test point in each interval and evaluating $f'$ at that point.

5. Apply the sign change conditions of the First Derivative Test to classify each critical point as a local maximum, local minimum, or neither.

6. If a local extremum exists at $c$, calculate the local extreme value by finding $f(c)$.

Second Derivative Test

The Second Derivative Test uses the value of the second derivative at a critical point where the first derivative is zero ($f'(c)=0$). It is often quicker when applicable, but it requires the existence of the second derivative.

Let $f$ be a function that is differentiable on an open interval $I$, and let $c$ be an interior point in $I$ such that $f'(c) = 0$. Assume the second derivative $f''(c)$ exists at $c$.

• Condition for Local Maximum: If $f''(c) < 0$, then $f$ has a local maximum at $c$. (The function's graph is concave down at $c$).
• Condition for Local Minimum: If $f''(c) > 0$, then $f$ has a local minimum at $c$. (The function's graph is concave up at $c$).
• When the Test Fails: If $f''(c) = 0$ or $f''(c)$ is undefined, the Second Derivative Test gives no conclusion about local extrema at $c$. In such cases, you must use the First Derivative Test to determine the nature of the critical point.

Procedure using Second Derivative Test:

1. Find the first derivative $f'(x)$ and the second derivative $f''(x)$.

2. Find the critical points $c$ by setting $f'(x) = 0$ and solving for $x$. This test *only* applies to critical points where $f'(c)=0$.

3. For each critical point $c$ found in step 2, evaluate the second derivative $f''(c)$.

4. Apply the conditions based on the sign of $f''(c)$:

• If $f''(c) < 0$, $f$ has a local maximum at $x=c$. The local maximum value is $f(c)$.
• If $f''(c) > 0$, $f$ has a local minimum at $x=c$. The local minimum value is $f(c)$.
• If $f''(c) = 0$ or $f''(c)$ is undefined, the test fails. Proceed to use the First Derivative Test for this specific critical point.

---

Answer:

---

Answer:

Practical Problems on Maxima and Minima

One of the most significant and practical applications of differential calculus is in solving optimization problems. These problems involve finding the maximum or minimum value of a certain quantity (like area, volume, profit, cost, time, distance, etc.) under given constraints. The ability to model such situations mathematically and then use derivatives to find the extreme values makes calculus an indispensable tool in various fields like engineering, economics, physics, and business. Solving these problems typically involves identifying the quantity to be optimized, expressing it as a function, and then finding the extrema of that function using the techniques discussed earlier (finding critical points, and applying the First or Second Derivative Test).

---

Steps to Solve Optimization Problems

Solving optimization problems using calculus generally follows a systematic approach:

1. Understand the Problem: Read the problem statement carefully to fully grasp what is being asked. Identify the quantity that needs to be maximised or minimised. Assign a symbol (variable) to this quantity, say $Q$. Identify all other relevant quantities and assign variables to them as well.

2. Draw a Diagram (if applicable): For geometric problems, drawing a clear diagram and labelling the relevant variables can greatly help in visualising the relationships between the quantities.

3. Find Relationships and Formulate the Function: Write down any equations that relate the variables identified in the problem. These usually come from the problem description (e.g., fixed sum, fixed perimeter, formulas for area/volume, etc.). Use these relationships to express the quantity $Q$ (which you want to optimize) as a function of a single independent variable, say $x$. For example, if $Q$ depends on $x$ and $y$, use a relation between $x$ and $y$ to express $y$ in terms of $x$ and substitute it into the expression for $Q$, resulting in $Q(x)$.

4. Determine the Domain: Identify the feasible range of the independent variable $x$ based on the constraints of the problem. For instance, lengths, widths, radii, etc., must typically be non-negative. This will define the domain (an interval) for the function $Q(x)$. This domain might be open or closed depending on the problem.

5. Find Critical Points: Calculate the first derivative of the function $Q(x)$, i.e., $Q'(x)$. Find the critical points within the determined domain by setting $Q'(x) = 0$ or by finding points in the domain where $Q'(x)$ is undefined.

6. Test Critical Points: Use either the First Derivative Test or the Second Derivative Test to classify each critical point found in step 5 as a local maximum, a local minimum, or neither. The First Derivative Test is generally more robust as it works even when the second derivative is zero or undefined. The Second Derivative Test is often quicker if the second derivative is easy to compute and is non-zero at the critical points.

7. Evaluate at Endpoints (if applicable): If the domain determined in step 4 is a closed interval $[a, b]$ and you are looking for absolute extrema, you must also evaluate the function $Q(x)$ at the endpoints $a$ and $b$, in addition to evaluating it at the critical points within $(a, b)$.

8. Identify the Optimal Value and Answer the Question: Based on the results of the tests in step 6 or the comparison of values in step 7 (for closed intervals), identify the value(s) of $x$ that yield the required maximum or minimum value of $Q$. State the answer clearly, providing both the value(s) of the independent variable(s) and the resulting maximum or minimum value of the quantity $Q$, ensuring units are included if applicable.

---

Answer:

---

Answer: