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Fundamental Properties of Differentiation

Michael Green

Michael Green

6 min read

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Study Guide Overview

This study guide covers derivatives of sine and cosine functions, including sin(x), cos(x), sin(kx), and cos(kx). It explains differentiation rules and the chain rule, provides worked examples, and offers practice questions. The guide also demonstrates deriving these derivatives using the formal definition of a derivative and includes a glossary of key terms.

Derivatives of Sine and Cosine Functions

Table of Contents

  1. Introduction
  2. Differentiating Sin x and Cos x
  3. Differentiating Sin kx and Cos kx
  4. Worked Examples
  5. Using the Definition of a Derivative
  6. Glossary
  7. Practice Questions
  8. Summary and Key Takeaways

Introduction

In calculus, understanding how to differentiate sine and cosine functions is fundamental. This guide covers the rules for differentiating these trigonometric functions, including their more complex forms, and provides worked examples and practice questions to help solidify your understanding.

Differentiating Sin x and Cos x

Key Concepts

  • If f(x)=sinxf(x) = \sin x,
Key Concept

then f(x)=cosxf'(x) = \cos x

  • If g(x)=cosxg(x) = \cos x,
Key Concept

then g(x)=sinxg'(x) = -\sin x

  • If h(x)=sinxh(x) = -\sin x,
Key Concept

then h(x)=cosxh'(x) = -\cos x

  • If j(x)=cosxj(x) = -\cos x,
Key Concept

then j(x)=sinxj'(x) = \sin x

  • This sequence then repeats.

Differentiating Sin kx and Cos kx

When dealing with functions of the form sin(kx)\sin(kx) and cos(kx)\cos(kx), the differentiation involves the chain rule.

Key Concepts

  • If f(x)=sin(kx)f(x) = \sin(kx),
Key Concept

then f(x)=kcos(kx)f'(x) = k \cos(kx)

  • If g(x)=cos(kx)g(x) = \cos(kx),
Key Concept

then g(x)=ksin(kx)g'(x) = -k \sin(kx)

Exam Tip

Remember that these results come from applying the chain rule.

Worked Examples

Example 1

Differentiate the following function: f(x)=cosx4sinxf(x) = \cos x - 4 \sin x

Solution: cosx\cos x differentiates to sinx-\sin x
sinx\sin x differentiates to cosx\cos x

So, f(x)=sinx4cosxf'(x) = -\sin x - 4 \cos x

Example 2

Differentiate the following function: g(x)=9sin(23x)2cos(3x)g(x) = 9 \sin \left(\frac{2}{3} x\right) - 2 \cos (3x)

Solution: sin(kx)\sin(kx) differentiates to kcos(kx)k \cos(kx)
cos(kx)\cos(kx) differentiates to ksin(kx)-k \sin(kx)

So, g(x)=923cos(23x)2(3sin(3x))g'(x) = 9 \cdot \frac{2}{3} \cos \left(\frac{2}{3} x\right) - 2 \left(-3 \sin(3x)\right)

Simplifying, g(x)=6cos(23x)+6sin(3x)=6(cos(23x)+sin(3x))g'(x) = 6 \cos \left(\frac{2}{3} x\right) + 6 \sin(3x) = 6\left(\cos \left(\frac{2}{3} x\right) + \sin(3x)\right)

Using the Definition of a Derivative

Definition

The derivative of a function f(x)f(x) can be defined as: f(x)=limh0f(x+h)f(x)hf'(x) = \lim_{h \to 0} \frac{f(x+h) - f(x)}{h}

Differentiating sinx\sin x Using the Definition

Given f(x)=sinxf(x) = \sin x: f(x)=limh0sin(x+h)sin(x)hf'(x) = \lim_{h \to 0} \frac{\sin(x+h) - \sin(x)}{h}

Using the addition formula: sin(A+B)=sinAcosB+cosAsinB\sin(A+B) = \sin A \cos B + \cos A \sin B

So, f(x)=limh0sinxcosh+cosxsinhsinxhf'(x) = \lim_{h \to 0} \frac{\sin x \cos h + \cos x \sin h - \sin x}{h}

Factorizing: f(x)=limh0(sinx(cosh1h)+cosx(sinhh))f'(x) = \lim_{h \to 0} \left(\sin x \left(\frac{\cos h - 1}{h}\right) + \cos x \left(\frac{\sin h}{h}\right)\right)

Applying the limits: limh0sinhh=1\lim_{h \to 0} \frac{\sin h}{h} = 1 and limh0cosh1h=0\lim_{h \to 0} \frac{\cos h - 1}{h} = 0

So, f(x)=sinx0+cosx1=cosxf'(x) = \sin x \cdot 0 + \cos x \cdot 1 = \cos x

Differentiating cosx\cos x Using the Definition

Given g(x)=cosxg(x) = \cos x: g(x)=limh0cos(x+h)cos(x)hg'(x) = \lim_{h \to 0} \frac{\cos(x+h) - \cos(x)}{h}

Using the addition formula: cos(A+B)=cosAcosBsinAsinB\cos(A+B) = \cos A \cos B - \sin A \sin B

So, g(x)=limh0cosxcoshsinxsinhcosxhg'(x) = \lim_{h \to 0} \frac{\cos x \cos h - \sin x \sin h - \cos x}{h}

Factorizing: g(x)=limh0(cosx(cosh1h)sinx(sinhh))g'(x) = \lim_{h \to 0} \left(\cos x \left(\frac{\cos h - 1}{h}\right) - \sin x \left(\frac{\sin h}{h}\right)\right)

Applying the limits: limh0sinhh=1\lim_{h \to 0} \frac{\sin h}{h} = 1 and limh0cosh1h=0\lim_{h \to 0} \frac{\cos h - 1}{h} = 0

So, g(x)=cosx0sinx1=sinxg'(x) = \cos x \cdot 0 - \sin x \cdot 1 = -\sin x

Glossary

  • Derivative: A measure of how a function changes as its input changes.
  • Chain Rule: A formula to compute the derivative of a composite function.
  • Trigonometric Addition Formulae: Formulas that express trigonometric functions of sums of angles in terms of products of trigonometric functions of individual angles.
  • Limit: The value that a function approaches as the input approaches some value.

Practice Questions

Practice Question
  1. Differentiate f(x)=5sin(2x)f(x) = 5 \sin(2x).

  2. Differentiate h(x)=4cos(3x)sin(5x)h(x) = 4 \cos(3x) - \sin(5x).

  3. Use the definition of a derivative to differentiate g(x)=sinxg(x) = \sin x.

Summary and Key Takeaways

  • The derivative of sinx\sin x is cosx\cos x.
  • The derivative of cosx\cos x is sinx-\sin x.
  • Using the chain rule, the derivatives of sin(kx)\sin(kx) and cos(kx)\cos(kx) are kcos(kx)k \cos(kx) and ksin(kx)-k \sin(kx), respectively.
  • Understanding and applying the definition of a derivative is crucial for deriving these results.

Key Takeaways

Key Concept

The derivative of sinx\sin x is cosx\cos x.

Key Concept

The derivative of cosx\cos x is sinx-\sin x.

Key Concept

Use the chain rule for functions involving constants, such as sin(kx)\sin(kx) and cos(kx)\cos(kx).

Key Concept

Master the trigonometric addition formulae and limit theorems for deeper understanding.

Key Concept

Practice differentiating using both rules and definitions to solidify understanding.

Exam Tip

Always double-check your differentiation, especially the signs when differentiating trigonometric functions.

For more complex functions, break them down into simpler components and apply differentiation rules step by step.

Question 1 of 11

What is the derivative of f(x)=sinxf(x) = \sin x ? 🚀

sinx-\sin x

cosx\cos x

cosx-\cos x

11