02 1-1 (b) Implicit differentiation
The method
- Differentiate both sides of the equation with respect to .
- Treat as a differentiable function of and apply the chain rule to every term containing .
- Collect all terms containing on one side, factorise, and divide.
The chain-rule step
- .
- and .
- (product rule).
- If you differentiate with respect to instead, the factor appears.
✍️ Worked examples
Example 1: Find if , then find the tangent and normal at .
Solution:
- Differentiate: .
- So .
- At the slope is .
- Tangent: , that is .
- Normal slope is the negative reciprocal, , so the normal is .
Example 2: Find for and evaluate it at .
Solution:
- Differentiate both sides: .
- Group: .
- So .
- At : .
Example 3: Find if .
Solution:
- Differentiate: .
- Group: .
- So .
Example 4: Find if .
Solution:
- Differentiate: .
- Group: .
- So .
Example 5: Find if .
Solution:
- Differentiate: .
- Group: .
- So .
Important remarks
- The final formula usually contains both and .
- To find the slope at a given you must first find the matching from the original equation.
- A point must lie on the curve before you substitute it; check it in the equation.
- A horizontal tangent needs the numerator of equal to zero; a vertical tangent needs the denominator equal to zero.