MTH165 – Mathematics for Engineers

Differential Calculus
and Its Applications

Unit II
Course Code: MTH165
Course Title: Mathematics for Engineers
Credits: 3 – 1 – 0 – 4
Weightage: ATT 5 | CA 25 | MTT 20 | ETT 50

Course Outcome CO2

Apply differential calculus techniques to evaluate derivatives, solve optimization problems, determine limits using L'Hospital's rule, and construct Taylor and Maclaurin series expansions.

📑 Table of Contents

IOrientation: The Differential Calculus Framework3
IIFundamental Differentiation4
IIIParametric Differentiation6
IVImplicit Differentiation7
VLogarithmic Differentiation8
VISuccessive Differentiation9
VIIRolle's Theorem10
VIIIMean Value Theorems (Lagrange & Cauchy)11
IXSeries Expansion: Taylor & Maclaurin Theorems12
XUndefined Limit Patterns (Indeterminate Forms)14
XIEvaluation of Indeterminate Limits (L'Hospital's Rule)15
XIIOptimization (Maxima and Minima)16
Comprehensive Summary Table & Formula Sheet18
Exam Tips, Practice Questions & Solutions19
References, Key Takeaways & CO Mapping22
How to use these notes: Each section contains theory, formulas, worked examples, and exam-focused tips. Use the summary table at the end for quick revision before exams.

I. Orientation: The Differential Calculus Framework

1.1 What is Differential Calculus?

Differential calculus studies instantaneous change through the derivative, developed systematically by Isaac Newton and Gottfried Wilhelm Leibniz in the 17th century.

For a function \( y = f(x) \), the derivative measures the limiting ratio of the change in \( y \) to the corresponding change in \( x \).

1.2 Definition of the Derivative

\[ \boxed{\,f'(x) \;=\; \frac{dy}{dx} \;=\; \lim_{h \to 0} \frac{f(x+h) - f(x)}{h}\,} \]
Notation & Terminology

1.3 Interpretations of the Derivative

InterpretationMeaning
Geometrical\( f'(a) \) = slope of the tangent to \( y = f(x) \) at \( (a, f(a)) \)
PhysicalIf \( s(t) \) = displacement, then \( \dfrac{ds}{dt} \) = velocity and \( \dfrac{d^2s}{dt^2} \) = acceleration
Rate of ChangeInstantaneous rate of change of \( y \) with respect to \( x \)
EconomicMarginal cost, marginal revenue, marginal profit

1.4 Differentiability Condition

A function is differentiable at \( x = a \) only when:

  1. Left derivative exists: \( f'(a^-) = \displaystyle\lim_{h \to 0^-} \frac{f(a+h) - f(a)}{h} \)
  2. Right derivative exists: \( f'(a^+) = \displaystyle\lim_{h \to 0^+} \frac{f(a+h) - f(a)}{h} \)
  3. Both are finite and equal: \( f'(a^-) = f'(a^+) \)

1.5 Continuity vs Differentiability

\[ \text{Differentiability} \;\Longrightarrow\; \text{Continuity} \] \[ \text{Continuity} \;\not\Longrightarrow\; \text{Differentiability} \]
Classic Example: \( f(x) = |x| \)

Reason: Left derivative = \(-1\), Right derivative = \(+1\), they are not equal.

1.6 Working Convention

Angles in trigonometric differentiation are measured in RADIANS. Standard formulas such as \( \dfrac{d}{dx}(\sin x) = \cos x \) depend on this convention.
Exam Tip: Always check continuity and differentiability separately for piecewise functions. A function can be continuous but not differentiable at a point.

II. Fundamental Differentiation

2.1 Derivatives of Standard Functions

Algebraic and Exponential Functions

FunctionDerivativeFunctionDerivative
\( c \) (constant)\( 0 \)\( x^n \)\( nx^{n-1} \)
\( e^x \)\( e^x \)\( a^x \)\( a^x \ln a \)
\( \ln x \)\( \dfrac{1}{x} \)\( \log_a x \)\( \dfrac{1}{x \ln a} \)
\( \sqrt{x} \)\( \dfrac{1}{2\sqrt{x}} \)\( \dfrac{1}{x} \)\( -\dfrac{1}{x^2} \)
Conditions
\( c \) = constant, \( n \) = real where defined, \( a > 0 \), \( a \neq 1 \)

Trigonometric Functions

FunctionDerivativeFunctionDerivative
\( \sin x \)\( \cos x \)\( \cos x \)\( -\sin x \)
\( \tan x \)\( \sec^2 x \)\( \cot x \)\( -\csc^2 x \)
\( \sec x \)\( \sec x \tan x \)\( \csc x \)\( -\csc x \cot x \)

Inverse Trigonometric Functions

FunctionDerivative
\( \sin^{-1}x \)\( \dfrac{1}{\sqrt{1-x^2}} \)
\( \cos^{-1}x \)\( -\dfrac{1}{\sqrt{1-x^2}} \)
\( \tan^{-1}x \)\( \dfrac{1}{1+x^2} \)
\( \cot^{-1}x \)\( -\dfrac{1}{1+x^2} \)
\( \sec^{-1}x \)\( \dfrac{1}{|x|\sqrt{x^2-1}} \)
\( \csc^{-1}x \)\( -\dfrac{1}{|x|\sqrt{x^2-1}} \)

Hyperbolic Functions

FunctionDerivativeFunctionDerivative
\( \sinh x \)\( \cosh x \)\( \cosh x \)\( \sinh x \)
\( \tanh x \)\( \text{sech}^2 x \)\( \coth x \)\( -\text{csch}^2 x \)

2.2 General Rules of Differentiation

For differentiable functions \( u(x) \) and \( v(x) \):

RuleFormula
Constant Multiple\( \dfrac{d}{dx}(ku) = k\dfrac{du}{dx} \)
Sum / Difference\( \dfrac{d}{dx}(u \pm v) = \dfrac{du}{dx} \pm \dfrac{dv}{dx} \)
Product Rule\( \dfrac{d}{dx}(uv) = u\dfrac{dv}{dx} + v\dfrac{du}{dx} \)
Quotient Rule\( \dfrac{d}{dx}\!\left(\dfrac{u}{v}\right) = \dfrac{v\frac{du}{dx} - u\frac{dv}{dx}}{v^2} \)
Chain Rule\( \dfrac{d}{dx}[f(g(x))] = f'(g(x)) \cdot g'(x) \)

2.3 Worked Examples

Example 1: Product Rule

Problem: Differentiate \( y = x^2 e^x \)

Solution:

\[ \frac{dy}{dx} = 2x \cdot e^x + x^2 \cdot e^x = e^x(x^2 + 2x) \]
Example 2: Quotient Rule

Problem: Differentiate \( y = \dfrac{\sin x}{x} \)

Solution:

\[ \frac{dy}{dx} = \frac{x \cos x - \sin x \cdot 1}{x^2} = \frac{x\cos x - \sin x}{x^2} \]
Example 3: Chain Rule

Problem: Differentiate \( y = \sin(x^3) \)

Solution:

\[ \frac{dy}{dx} = \cos(x^3) \cdot 3x^2 = 3x^2\cos(x^3) \]

III. Parametric Differentiation

3.1 Derivatives of Parametric Forms

When \( x \) and \( y \) are expressed through a parameter \( t \):

First Derivative

\[ \boxed{\frac{dy}{dx} = \frac{dy/dt}{dx/dt} \qquad \text{provided } \frac{dx}{dt} \neq 0} \]

Second Derivative

\[ \boxed{\frac{d^2y}{dx^2} = \frac{\dfrac{d}{dt}\!\left(\dfrac{dy}{dx}\right)}{\dfrac{dx}{dt}}} \]
Differentiating \( \dfrac{dy}{dx} \) directly with respect to \( t \) alone does NOT produce \( \dfrac{d^2y}{dx^2} \). You must divide by \( \dfrac{dx}{dt} \).

3.2 Worked Examples

Example 1: Basic Parametric

Problem: If \( x = t^2 \) and \( y = t^3 \), find \( \dfrac{dy}{dx} \) and \( \dfrac{d^2y}{dx^2} \)

Solution:

Step 1: \( \dfrac{dx}{dt} = 2t \), \( \dfrac{dy}{dt} = 3t^2 \)

Step 2: First derivative

\[ \frac{dy}{dx} = \frac{3t^2}{2t} = \frac{3t}{2} \]

Step 3: Second derivative

\[ \frac{d^2y}{dx^2} = \frac{\dfrac{d}{dt}\!\left(\dfrac{3t}{2}\right)}{\dfrac{dx}{dt}} = \frac{\dfrac{3}{2}}{2t} = \frac{3}{4t} \]
Example 2: Trigonometric Parametric

Problem: If \( x = a\cos\theta \), \( y = a\sin\theta \), find \( \dfrac{dy}{dx} \)

Solution:

\[ \frac{dx}{d\theta} = -a\sin\theta, \quad \frac{dy}{d\theta} = a\cos\theta \] \[ \frac{dy}{dx} = \frac{a\cos\theta}{-a\sin\theta} = -\cot\theta \]

IV. Implicit Differentiation

4.1 Derivatives of Implicit Functions

Implicit differentiation applies when an equation relates \( x \) and \( y \) without isolating \( y \).

Principle

Differentiate both sides with respect to \( x \), treating \( y \) as \( y(x) \). By the chain rule:

\[ \frac{d}{dx}(y^n) = ny^{n-1}\frac{dy}{dx} \]

General Formula

If \( F(x, y) = 0 \) and \( F_y \neq 0 \):

\[ \boxed{\frac{dy}{dx} = -\frac{F_x}{F_y}} \]

Where \( F_x \) and \( F_y \) are partial derivatives of \( F \) with respect to \( x \) and \( y \).

4.2 Worked Examples

Example 1: Circle

Problem: Find \( \dfrac{dy}{dx} \) for \( x^2 + y^2 = 25 \)

Solution:

\[ 2x + 2y\frac{dy}{dx} = 0 \] \[ \frac{dy}{dx} = -\frac{x}{y} \]

Thus, the tangent slope depends on both coordinates of the point.

Example 2: Higher Degree

Problem: If \( x^3 + y^3 = 3axy \), find \( \dfrac{dy}{dx} \)

Solution:

\[ 3x^2 + 3y^2\frac{dy}{dx} = 3a\left(y + x\frac{dy}{dx}\right) \] \[ 3x^2 + 3y^2\frac{dy}{dx} = 3ay + 3ax\frac{dy}{dx} \] \[ 3y^2\frac{dy}{dx} - 3ax\frac{dy}{dx} = 3ay - 3x^2 \] \[ \frac{dy}{dx} = \frac{ay - x^2}{y^2 - ax} \]

V. Logarithmic Differentiation

5.1 Logarithmic Method

Logarithmic differentiation simplifies:

Method

  1. Take natural logarithms on both sides
  2. Use logarithm laws to simplify
  3. Differentiate implicitly
  4. Multiply by \( y \) to recover \( \dfrac{dy}{dx} \)

5.2 Variable Power Rule

For positive \( u(x) \) and differentiable \( v(x) \), if \( y = u^v \):

\[ \ln y = v \ln u \] \[ \frac{y'}{y} = v'\ln u + v\frac{u'}{u} \] \[ \boxed{y' = u^v\!\left[v'\ln u + v\frac{u'}{u}\right]} \]

5.3 Worked Examples

Example 1: \( y = x^x \)

Problem: Differentiate \( y = x^x \) (where \( x > 0 \))

Solution:

\[ \ln y = x \ln x \] \[ \frac{y'}{y} = \ln x + 1 \] \[ y' = x^x(\ln x + 1) \]
Example 2: \( y = x^{\sin x} \)

Problem: Differentiate \( y = x^{\sin x} \)

Solution:

\[ \ln y = \sin x \cdot \ln x \] \[ \frac{y'}{y} = \cos x \cdot \ln x + \sin x \cdot \frac{1}{x} \] \[ y' = x^{\sin x}\left[\cos x \ln x + \frac{\sin x}{x}\right] \]

VI. Successive Differentiation

6.1 Higher Order Derivatives

Higher-order derivatives describe repeated rates of change.

Notation: \( f''(x) \), \( f^{(n)}(x) \), or \( \dfrac{d^ny}{dx^n} \)

6.2 Standard Patterns

\[ \boxed{\frac{d^n}{dx^n}(x^m) = \frac{m!}{(m-n)!}x^{m-n} \qquad \text{for } n \leq m} \] \[ \boxed{\frac{d^n}{dx^n}(e^{ax}) = a^n e^{ax}} \] \[ \boxed{\frac{d^n}{dx^n}(\sin ax) = a^n \sin\!\left(ax + \frac{n\pi}{2}\right)} \] \[ \boxed{\frac{d^n}{dx^n}(\cos ax) = a^n \cos\!\left(ax + \frac{n\pi}{2}\right)} \] \[ \boxed{\frac{d^n}{dx^n}(a^x) = a^x (\ln a)^n} \] \[ \boxed{\frac{d^n}{dx^n}(\ln x) = \frac{(-1)^{n-1}(n-1)!}{x^n}} \]

Where: \( m, n \) are non-negative integers and \( a \) is constant.

6.3 Polynomial Termination

If \( n > m \), then the \( n \)th derivative of \( x^m \) is zero.

\[ \frac{d^n}{dx^n}(x^m) = 0 \qquad \text{for } n > m \]

6.4 Engineering Interpretation

For position \( s(t) \):

DerivativePhysical Meaning
\( s'(t) \)Velocity
\( s''(t) \)Acceleration
\( s'''(t) \)Jerk
\( s^{(4)}(t) \)Snap / Jounce

6.5 Worked Examples

Example 1: Exponential

Problem: If \( y = e^{2x} \), find \( y''' \)

Solution:

\[ y''' = 2^3 e^{2x} = 8e^{2x} \]
Example 2: Trigonometric

Problem: Find the \( n \)th derivative of \( y = \sin(3x) \)

Solution:

\[ \frac{d^ny}{dx^n} = 3^n \sin\!\left(3x + \frac{n\pi}{2}\right) \]

VII. Rolle's Theorem

7.1 Statement

If \( f \) is:

  1. Continuous on \( [a, b] \)
  2. Differentiable on \( (a, b) \)
  3. \( f(a) = f(b) \)

Then there exists at least one \( c \in (a, b) \) such that:

\[ \boxed{f'(c) = 0} \]

7.2 Geometrical Meaning

A smooth curve joining two points at the same height must have at least one tangent parallel to the x-axis.

7.3 Worked Examples

Example 1: Quadratic

Problem: Verify Rolle's theorem for \( f(x) = x^2 - 4x + 3 \) on \( [1, 3] \)

Solution:

Step 1: \( f(1) = 1 - 4 + 3 = 0 \), \( f(3) = 9 - 12 + 3 = 0 \) ✓

Step 2: \( f'(x) = 2x - 4 = 0 \)

Step 3: \( x = 2 \), and \( 2 \in (1, 3) \) ✓

Therefore: \( c = 2 \)

Example 2: Cubic

Problem: Verify Rolle's theorem for \( f(x) = x^3 - 3x^2 + 2x \) on \( [0, 2] \)

Solution:

Step 1: \( f(0) = 0 \), \( f(2) = 8 - 12 + 4 = 0 \) ✓

Step 2: \( f'(x) = 3x^2 - 6x + 2 = 0 \)

Step 3: \( x = \dfrac{6 \pm \sqrt{36 - 24}}{6} = \dfrac{6 \pm \sqrt{12}}{6} = 1 \pm \dfrac{1}{\sqrt{3}} \)

Both values \( 1 - \dfrac{1}{\sqrt{3}} \approx 0.42 \) and \( 1 + \dfrac{1}{\sqrt{3}} \approx 1.58 \) lie in \( (0, 2) \) ✓

7.4 Limitation

The theorem guarantees existence, not uniqueness. Failure of any hypothesis removes the guarantee.

VIII. Mean Value Theorems

8.1 Lagrange's Mean Value Theorem (LMVT)

If \( f \) is:

  1. Continuous on \( [a, b] \)
  2. Differentiable on \( (a, b) \)

Then there exists some \( c \in (a, b) \) such that:

\[ \boxed{f'(c) = \frac{f(b) - f(a)}{b - a}} \]

Geometrical Meaning: The tangent at \( c \) is parallel to the secant joining the endpoints.

8.2 Cauchy's Mean Value Theorem (CMVT)

If \( f \) and \( g \) are:

  1. Continuous on \( [a, b] \)
  2. Differentiable on \( (a, b) \)
  3. \( g'(c) \neq 0 \)

Then there exists some \( c \in (a, b) \) such that:

\[ \boxed{\frac{f'(c)}{g'(c)} = \frac{f(b) - f(a)}{g(b) - g(a)}} \]

8.3 Relationship Between Theorems

TheoremRelationship
Rolle's TheoremEqual-endpoint case of Lagrange's theorem
Lagrange's TheoremFollows from Cauchy's theorem by taking \( g(x) = x \)

8.4 Worked Examples

Example 1: LMVT

Problem: Verify LMVT for \( f(x) = x^2 \) on \( [1, 3] \)

Solution:

Step 1: Average slope:

\[ \frac{f(3) - f(1)}{3 - 1} = \frac{9 - 1}{2} = 4 \]

Step 2: \( f'(c) = 2c = 4 \)

Step 3: \( c = 2 \), and \( 2 \in (1, 3) \) ✓

Example 2: LMVT Application

Problem: Verify LMVT for \( f(x) = x^3 - 3x \) on \( [0, 2] \)

Solution:

Average slope \( = \dfrac{f(2) - f(0)}{2 - 0} = \dfrac{(8-6) - 0}{2} = 1 \)

\( f'(c) = 3c^2 - 3 = 1 \Rightarrow 3c^2 = 4 \Rightarrow c = \dfrac{2}{\sqrt{3}} \approx 1.15 \in (0, 2) \) ✓

IX. Series Expansion Theorems

9.1 Taylor's Theorem

Taylor's theorem represents a sufficiently differentiable function near a chosen point.

Taylor Expansion About \( x = a \):

\[ \boxed{f(x) = f(a) + (x-a)f'(a) + \frac{(x-a)^2}{2!}f''(a) + \cdots + R_n} \]

Lagrange Form of Remainder:

\[ \boxed{R_n = \frac{f^{(n+1)}(\xi)\,(x-a)^{n+1}}{(n+1)!}} \]

Where \( \xi \) lies between \( a \) and \( x \).

9.2 Maclaurin's Theorem

Setting \( a = 0 \) in Taylor's theorem gives Maclaurin's expansion:

\[ \boxed{f(x) = f(0) + xf'(0) + \frac{x^2}{2!}f''(0) + \cdots} \]

9.3 Standard Series Expansions

\[ \boxed{e^x = 1 + x + \frac{x^2}{2!} + \frac{x^3}{3!} + \cdots} \] \[ \boxed{\sin x = x - \frac{x^3}{3!} + \frac{x^5}{5!} - \cdots} \] \[ \boxed{\cos x = 1 - \frac{x^2}{2!} + \frac{x^4}{4!} - \cdots} \] \[ \boxed{\ln(1+x) = x - \frac{x^2}{2} + \frac{x^3}{3} - \cdots \qquad \text{for } |x| < 1} \] \[ \boxed{\tan^{-1}x = x - \frac{x^3}{3} + \frac{x^5}{5} - \cdots \qquad \text{for } |x| \leq 1} \] \[ \boxed{(1+x)^n = 1 + nx + \frac{n(n-1)}{2!}x^2 + \cdots} \]

9.4 Worked Examples

Example 1: Maclaurin Series for \( e^x \)

Problem: Expand \( e^x \) using Maclaurin's series

Solution:

\( f(x) = e^x \Rightarrow f(0) = 1 \)

\( f'(x) = e^x \Rightarrow f'(0) = 1 \)

\( f''(x) = e^x \Rightarrow f''(0) = 1 \)

All derivatives at 0 equal 1. Therefore:

\[ e^x = 1 + x + \frac{x^2}{2!} + \frac{x^3}{3!} + \cdots \]
Example 2: Maclaurin Series for \( \sin x \)

Problem: Expand \( \sin x \) using Maclaurin's series

Solution:

\( f(0) = 0 \), \( f'(0) = 1 \), \( f''(0) = 0 \), \( f'''(0) = -1 \), \( f^{(4)}(0) = 0 \), ...

\[ \sin x = x - \frac{x^3}{3!} + \frac{x^5}{5!} - \cdots \]
Example 3: Taylor Series about \( x = 1 \)

Problem: Expand \( \ln x \) about \( x = 1 \) up to 3 terms

Solution:

\( f(x) = \ln x \), \( f(1) = 0 \)

\( f'(x) = \dfrac{1}{x} \Rightarrow f'(1) = 1 \)

\( f''(x) = -\dfrac{1}{x^2} \Rightarrow f''(1) = -1 \)

\( f'''(x) = \dfrac{2}{x^3} \Rightarrow f'''(1) = 2 \)

\[ \ln x = (x-1) - \frac{(x-1)^2}{2} + \frac{(x-1)^3}{3} - \cdots \]

9.5 Application

Truncated series provide local polynomial approximations:

X. Undefined Limit Patterns

10.1 Indeterminate Forms

An indeterminate form is a limit pattern whose appearance alone does not determine the limit's value.

Principal Indeterminate Forms:

\[ \boxed{\frac{0}{0}, \quad \frac{\infty}{\infty}, \quad 0 \cdot \infty, \quad \infty - \infty, \quad 0^0, \quad 1^\infty, \quad \infty^0} \]

10.2 Why Indeterminate?

The form \( \dfrac{0}{0} \) can produce different results:

ExpressionLimit as \( x \to 0 \)
\( \dfrac{x}{x} \)\( 1 \)
\( \dfrac{x^2}{x} \)\( 0 \)
\( \dfrac{x}{x^2} \)Unbounded (\( \infty \))

10.3 Transformations

FormTransformation
\( 0 \cdot \infty \)Convert into a quotient: \( \dfrac{f}{1/g} \) or \( \dfrac{g}{1/f} \)
\( \infty - \infty \)Combine using common denominator or rationalization
\( 0^0, \; 1^\infty, \; \infty^0 \)Set \( y = f(x)^{g(x)} \), then examine \( \ln y = g(x)\ln f(x) \)

10.4 Distinction

Expressions such as \( \dfrac{1}{0} \) are undefined or divergent patterns, NOT automatically indeterminate forms.

XI. Evaluation of Indeterminate Limits

11.1 L'Hospital's Rule

Statement

If \( f \) and \( g \) are differentiable near \( a \), \( g'(x) \neq 0 \), and \( \dfrac{f}{g} \) has form \( \dfrac{0}{0} \) or \( \dfrac{\infty}{\infty} \), then:

\[ \boxed{\lim_{x \to a} \frac{f(x)}{g(x)} = \lim_{x \to a} \frac{f'(x)}{g'(x)}} \]

Provided the limit on the right exists or is infinite.

11.2 Repeated Use

The rule may be applied again if the differentiated quotient remains \( \dfrac{0}{0} \) or \( \dfrac{\infty}{\infty} \).

11.3 Worked Examples

Example 1: \( \frac{0}{0} \) Form

Problem: Evaluate \( \displaystyle\lim_{x \to 0} \frac{e^x - 1 - x}{x^2} \)

Solution:

Step 1: Form is \( \dfrac{0}{0} \) ✓

Step 2: Apply L'Hospital's rule:

\[ = \lim_{x \to 0} \frac{e^x - 1}{2x} \]

Step 3: Still \( \dfrac{0}{0} \), apply again:

\[ = \lim_{x \to 0} \frac{e^x}{2} = \frac{1}{2} \]
Example 2: \( \frac{\infty}{\infty} \) Form

Problem: Evaluate \( \displaystyle\lim_{x \to \infty} \frac{\ln x}{x} \)

Solution:

\[ \lim_{x \to \infty} \frac{\ln x}{x} = \lim_{x \to \infty} \frac{1/x}{1} = \lim_{x \to \infty} \frac{1}{x} = 0 \]
Example 3: \( 0 \cdot \infty \) Form

Problem: Evaluate \( \displaystyle\lim_{x \to 0^+} x \ln x \)

Solution:

Rewrite as \( \dfrac{\ln x}{1/x} \) (form \( \dfrac{\infty}{\infty} \)):

\[ = \lim_{x \to 0^+} \frac{1/x}{-1/x^2} = \lim_{x \to 0^+} (-x) = 0 \]

11.4 Restriction

The derivative of a quotient is NOT being calculated. Numerator and denominator are differentiated separately only within the limit theorem.

XII. Optimization (Maxima and Minima)

12.1 Critical Points

Interior candidates occur where:

Provided \( f(c) \) exists.

12.2 First Derivative Test

Sign Change of \( f' \) at \( c \)Conclusion
Positive → NegativeLocal Maximum
Negative → PositiveLocal Minimum
No sign changeNo extremum established

12.3 Second Derivative Test

If \( f'(c) = 0 \):

ConditionConclusion
\( f''(c) > 0 \)Local Minimum
\( f''(c) < 0 \)Local Maximum
\( f''(c) = 0 \)Inconclusive – use first derivative test

12.4 Absolute Extrema on Closed Interval

On a closed interval \( [a, b] \):

  1. Find all critical points in \( (a, b) \)
  2. Evaluate \( f \) at all critical points
  3. Evaluate \( f(a) \) and \( f(b) \)
  4. Compare all values
Largest value = Absolute Maximum
Smallest value = Absolute Minimum

12.5 Worked Examples

Example 1: Quadratic

Problem: Find the extrema of \( f(x) = x^2 - 6x + 5 \)

Solution:

Step 1: \( f'(x) = 2x - 6 = 0 \implies x = 3 \)

Step 2: \( f''(x) = 2 > 0 \implies \) minimum

Step 3: \( f(3) = 9 - 18 + 5 = -4 \)

Conclusion: Absolute minimum value \( -4 \) at \( x = 3 \), with no finite maximum on the real line.

Example 2: Cubic

Problem: Find the maximum and minimum values of \( f(x) = 2x^3 - 15x^2 + 36x + 10 \)

Solution:

Step 1: \( f'(x) = 6x^2 - 30x + 36 = 0 \)

\( \Rightarrow x^2 - 5x + 6 = 0 \Rightarrow (x-2)(x-3) = 0 \Rightarrow x = 2, 3 \)

Step 2: \( f''(x) = 12x - 30 \)

\( f''(2) = 24 - 30 = -6 < 0 \Rightarrow \) local maximum at \( x = 2 \)

\( f''(3) = 36 - 30 = 6 > 0 \Rightarrow \) local minimum at \( x = 3 \)

Step 3: \( f(2) = 16 - 60 + 72 + 10 = 38 \) (max)

\( f(3) = 54 - 135 + 108 + 10 = 37 \) (min)

Example 3: Absolute Extrema

Problem: Find absolute extrema of \( f(x) = x^3 - 3x^2 + 1 \) on \( [-1, 4] \)

Solution:

Step 1: \( f'(x) = 3x^2 - 6x = 3x(x-2) = 0 \Rightarrow x = 0, 2 \)

Step 2: Evaluate:

\( f(-1) = -1 - 3 + 1 = -3 \)

\( f(0) = 1 \)

\( f(2) = 8 - 12 + 1 = -3 \)

\( f(4) = 64 - 48 + 1 = 17 \)

Step 3: Absolute Maximum = \( 17 \) at \( x = 4 \)

Absolute Minimum = \( -3 \) at \( x = -1 \) and \( x = 2 \)

📊 Comprehensive Summary Table

TopicKey Formula / Concept
Derivative Definition\( f'(x) = \displaystyle\lim_{h \to 0} \frac{f(x+h)-f(x)}{h} \)
Product Rule\( (uv)' = u'v + uv' \)
Quotient Rule\( \left(\dfrac{u}{v}\right)' = \dfrac{vu' - uv'}{v^2} \)
Chain Rule\( \dfrac{d}{dx}[f(g(x))] = f'(g(x))g'(x) \)
Parametric 1st Derivative\( \dfrac{dy}{dx} = \dfrac{dy/dt}{dx/dt} \)
Parametric 2nd Derivative\( \dfrac{d^2y}{dx^2} = \dfrac{d/dt(dy/dx)}{dx/dt} \)
Implicit Differentiation\( \dfrac{dy}{dx} = -\dfrac{F_x}{F_y} \)
Logarithmic Differentiation\( y' = u^v\!\left[v'\ln u + v\dfrac{u'}{u}\right] \)
\( n \)th Derivative of \( x^m \)\( \dfrac{m!}{(m-n)!}x^{m-n} \)
\( n \)th Derivative of \( e^{ax} \)\( a^n e^{ax} \)
\( n \)th Derivative of \( \sin ax \)\( a^n \sin(ax + n\pi/2) \)
\( n \)th Derivative of \( \cos ax \)\( a^n \cos(ax + n\pi/2) \)
Rolle's Theorem\( f'(c) = 0 \) when \( f(a) = f(b) \)
Lagrange's MVT\( f'(c) = \dfrac{f(b)-f(a)}{b-a} \)
Cauchy's MVT\( \dfrac{f'(c)}{g'(c)} = \dfrac{f(b)-f(a)}{g(b)-g(a)} \)
Taylor Series\( f(x) = \sum \dfrac{(x-a)^n}{n!}f^{(n)}(a) \)
Maclaurin Series\( f(x) = \sum \dfrac{x^n}{n!}f^{(n)}(0) \)
L'Hospital's Rule\( \lim \dfrac{f}{g} = \lim \dfrac{f'}{g'} \) for \( \dfrac{0}{0} \) or \( \dfrac{\infty}{\infty} \)
Second Derivative Test\( f''(c) > 0 \): min, \( f''(c) < 0 \): max

📐 Complete Standard Series Sheet

FunctionMaclaurin SeriesValid For
\( e^x \)\( 1 + x + \dfrac{x^2}{2!} + \dfrac{x^3}{3!} + \cdots \)All \( x \)
\( \sin x \)\( x - \dfrac{x^3}{3!} + \dfrac{x^5}{5!} - \cdots \)All \( x \)
\( \cos x \)\( 1 - \dfrac{x^2}{2!} + \dfrac{x^4}{4!} - \cdots \)All \( x \)
\( \ln(1+x) \)\( x - \dfrac{x^2}{2} + \dfrac{x^3}{3} - \cdots \)\( |x| < 1 \)
\( \tan^{-1}x \)\( x - \dfrac{x^3}{3} + \dfrac{x^5}{5} - \cdots \)\( |x| \leq 1 \)
\( (1+x)^n \)\( 1 + nx + \dfrac{n(n-1)}{2!}x^2 + \cdots \)\( |x| < 1 \)

🎯 Exam-Focused Tips

  1. Always check differentiability before applying theorems
  2. For parametric second derivative, remember to divide by \( \dfrac{dx}{dt} \)
  3. For implicit differentiation, treat \( y \) as a function of \( x \)
  4. Logarithmic differentiation is essential for variable powers \( u^v \)
  5. For L'Hospital's rule, verify the indeterminate form first
  6. For optimization, always check endpoints for absolute extrema
  7. Standard series should be memorized for quick expansion problems
  8. Rolle's theorem is a special case of LMVT – use it when \( f(a) = f(b) \)
  9. For second derivative test, if \( f''(c) = 0 \), fall back to first derivative test
  10. Always write the formula before substituting values in exam answers

📝 Practice Questions

Q1 Parametric

Find \( \dfrac{dy}{dx} \) if \( x = t + \dfrac{1}{t} \), \( y = t - \dfrac{1}{t} \)

Q2 Logarithmic

Differentiate \( y = x^{\sin x} \) using logarithmic differentiation

Q3 Rolle's

Verify Rolle's theorem for \( f(x) = x^3 - 3x^2 + 2x \) on \( [0, 2] \)

Q4 L'Hospital

Evaluate \( \displaystyle\lim_{x \to 0} \frac{\tan x - x}{x^3} \)

Q5 Optimization

Find the maximum and minimum values of \( f(x) = 2x^3 - 15x^2 + 36x + 10 \)

Q6 Series

Expand \( \ln(1+x) \) in powers of \( x \) using Maclaurin's series

Q7 Successive

Find the \( n \)th derivative of \( y = \sin(3x) \)

Q8 Implicit

If \( x^3 + y^3 = 3axy \), find \( \dfrac{dy}{dx} \)

✅ Practice Question Solutions

Solution Q1

\( x = t + \dfrac{1}{t} \), \( y = t - \dfrac{1}{t} \)

\[ \frac{dx}{dt} = 1 - \frac{1}{t^2}, \quad \frac{dy}{dt} = 1 + \frac{1}{t^2} \] \[ \frac{dy}{dx} = \frac{1 + 1/t^2}{1 - 1/t^2} = \frac{t^2 + 1}{t^2 - 1} \]
Solution Q2

\( y = x^{\sin x} \)

\[ \ln y = \sin x \cdot \ln x \] \[ \frac{y'}{y} = \cos x \ln x + \frac{\sin x}{x} \] \[ y' = x^{\sin x}\left[\cos x \ln x + \frac{\sin x}{x}\right] \]
Solution Q3

\( f(x) = x^3 - 3x^2 + 2x \) on \( [0, 2] \)

\( f(0) = 0 \), \( f(2) = 8 - 12 + 4 = 0 \) ✓

\( f'(x) = 3x^2 - 6x + 2 = 0 \)

\( x = 1 \pm \dfrac{1}{\sqrt{3}} \approx 0.42, 1.58 \) — both in \( (0, 2) \) ✓

Solution Q4

\( \displaystyle\lim_{x \to 0} \frac{\tan x - x}{x^3} \)

Form \( \dfrac{0}{0} \). Apply L'Hospital:

\[ = \lim_{x \to 0} \frac{\sec^2 x - 1}{3x^2} = \lim_{x \to 0} \frac{\tan^2 x}{3x^2} \] \[ = \lim_{x \to 0} \frac{1}{3}\left(\frac{\tan x}{x}\right)^2 = \frac{1}{3} \]
Solution Q5

\( f(x) = 2x^3 - 15x^2 + 36x + 10 \)

\( f'(x) = 6x^2 - 30x + 36 = 0 \Rightarrow x = 2, 3 \)

\( f''(x) = 12x - 30 \)

\( f''(2) = -6 < 0 \Rightarrow \) max at \( x = 2 \), \( f(2) = 38 \)

\( f''(3) = 6 > 0 \Rightarrow \) min at \( x = 3 \), \( f(3) = 37 \)

Solution Q6

\( \ln(1+x) = x - \dfrac{x^2}{2} + \dfrac{x^3}{3} - \cdots \) for \( |x| < 1 \)

Solution Q7

\( y = \sin(3x) \)

\[ \frac{d^ny}{dx^n} = 3^n \sin\!\left(3x + \frac{n\pi}{2}\right) \]
Solution Q8

\( x^3 + y^3 = 3axy \)

\[ 3x^2 + 3y^2\frac{dy}{dx} = 3a\left(y + x\frac{dy}{dx}\right) \] \[ \frac{dy}{dx} = \frac{ay - x^2}{y^2 - ax} \]

📚 Textbook References

ReferenceBook
T-1Advanced Engineering Mathematics – R.K. Jain, S.R.K. Iyengar (Narosa Publishing House)
R-1Higher Engineering Mathematics – B.S. Grewal (Khanna Publishers)
R-2Mathematics Textbook for Class XII Part I – NCERT
R-3Mathematics Textbook for Class XII Part II – NCERT
OR-1https://ncert.nic.in/textbook.php?lemh1=5-6
OR-2https://ncert.nic.in/textbook.php?lemh2=1-7

🏆 Key Takeaways

  1. Derivative = instantaneous rate of change = slope of tangent
  2. Differentiability implies continuity, but not vice versa
  3. Chain rule is the most powerful differentiation tool
  4. Parametric differentiation requires dividing by \( \dfrac{dx}{dt} \)
  5. Mean value theorems connect average and instantaneous change
  6. Taylor/Maclaurin series provide polynomial approximations
  7. L'Hospital's rule resolves indeterminate forms \( \dfrac{0}{0} \) and \( \dfrac{\infty}{\infty} \)
  8. Optimization uses first and second derivative tests
  9. Always verify hypotheses before applying any theorem
  10. Practice standard series for quick expansion problems

🎯 Course Outcome CO2 Mapping

After studying this unit, students should be able to:

CO2 ComponentsCovered in Section
Evaluate derivativesII, III, IV, V, VI
Solve optimization problemsXII
Determine limits using L'Hospital's ruleXI
Construct Taylor and Maclaurin seriesIX

⭐ End of Unit II Notes ⭐

Differential Calculus and Its Applications

MTH165 – Mathematics for Engineers

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