Grade 12 Mathematics
Grade 12 · Term 2Mathematics

Calculus

We introduce the concept of the derivative from first principles and apply differentiation rules to polynomial functions. We find equations of tangents and normals.

Week 5

6.1 Differentiation — Rules & Tangent Lines

  • Determine the derivative from first principles: $f'(x)=\lim_{h\to0}\frac{f(x+h)-f(x)}{h}$
  • Apply differentiation rules: constant, power, sum/difference
  • Find equations of tangent and normal lines to curves
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Real-World Connection

The speedometer in a car measures the DERIVATIVE of position with respect to time — how quickly position changes each instant. The derivative is the mathematical formalisation of 'instantaneous rate of change', which appears everywhere from physics to economics.

Definition

Derivative (First Principles)

The derivative of ff at xx is the limiting value of the average rate of change as the interval shrinks to zero.

f(x)=limh0f(x+h)f(x)hf'(x) = \lim_{h\to0}\frac{f(x+h)-f(x)}{h}

Power Rule

ddx(xn)=nxn1\frac{d}{dx}(x^n) = nx^{n-1}

Works for any real $n$

Constant Rule

ddx(k)=0\frac{d}{dx}(k) = 0

Derivative of any constant is 0

Sum/Difference Rule

ddx[f(x)±g(x)]=f(x)±g(x)\frac{d}{dx}[f(x)\pm g(x)] = f'(x)\pm g'(x)

Differentiate term by term

Property / Rule

Tangent and Normal Lines

The gradient of the tangent to y=f(x)y=f(x) at x=ax=a is f(a)f'(a). The normal is perpendicular to the tangent, so its gradient is 1f(a)-\frac{1}{f'(a)}.

mtangent=f(a);mnormal=1f(a)m_{\text{tangent}}=f'(a);\quad m_{\text{normal}}=-\frac{1}{f'(a)}

🚨 Common Mistake

Before differentiating, SIMPLIFY the expression. f(x)=x3+2xxf(x)=\frac{x^3+2x}{x} should be simplified to f(x)=x2+2f(x)=x^2+2 first. The power rule applies ONLY to individual power terms.

Worked Examples

Worked Example

Derivative from first principles

Problem

Find f(x)f'(x) from first principles if f(x)=3x2xf(x)=3x^2-x.

Worked Example

Tangent line to a curve

Problem

Find the equation of the tangent to f(x)=x34x+1f(x)=x^3-4x+1 at x=2x=2.
Activity — 8 Questions

CAPS Cognitive Level Distribution

L1 · Knowledge2 Q
L2 · Routine Procedures2 Q
L3 · Complex Procedures2 Q
L4 · Problem Solving2 Q
1
L1 · Knowledge2 marks
Differentiate f(x)=5x3f(x)=5x^3.
2
L1 · Knowledge1 mark
Differentiate y=7y=7.
3
L2 · Routine Procedures3 marks
Differentiate g(x)=4x43x2+2x7g(x)=4x^4-3x^2+2x-7.
4
L2 · Routine Procedures2 marks
Find f(x)f'(x) if f(x)=x=x1/2f(x)=\sqrt{x}=x^{1/2}.
5
L3 · Complex Procedures3 marks
Find the gradient of the tangent to y=x32xy=x^3-2x at the point where x=1x=-1.
6
L3 · Complex Procedures4 marks
Find the equation of the normal to f(x)=x2+3xf(x)=x^2+3x at x=1x=1.
7
L4 · Problem Solving4 marks
Find the point on y=x2y=x^2 where the tangent is parallel to y=4x1y=4x-1.
8
L4 · Problem Solving5 marks
Find f(x)f'(x) from first principles for f(x)=1xf(x)=\dfrac{1}{x}.
Calculus Grade 12 Maths CAPS Notes & Examples | MathSciBuddy