Stationary point: where dxdyβ=0. The curve has zero gradient (horizontal tangent).
Could be:
- MINIMUM (low point, curve goes up either side).
- MAXIMUM (high point, curve goes down either side).
- POINT OF INFLECTION (rare at IGCSE).
Method:
- Differentiate y to get dxdyβ.
- Set dxdyβ=0. Solve for x.
- Substitute back into ORIGINAL y equation to find y.
- Determine nature.
Determining nature.
Method A: Second derivative test.
- If dx2d2yβ>0 at the point: MINIMUM.
- If dx2d2yβ<0: MAXIMUM.
- If =0: inconclusive (rare at IGCSE).
Method B: Sign change test.
- Test dxdyβ either side of the point.
- β then +: MINIMUM.
- + then β: MAXIMUM.
- Same sign: point of inflection.
Worked example. y=x3β3x2β9x+5.
- dxdyβ=3x2β6xβ9.
- Set =0: x2β2xβ3=0. Factor: (xβ3)(x+1)=0. x=3 or x=β1.
- Find y:
- x=3: 27β27β27+5=β22. Point: (3,β22).
- x=β1: β1β3+9+5=10. Point: (β1,10).
- Second derivative: dx2d2yβ=6xβ6.
- At x=3: 12>0 β MIN.
- At x=β1: β12<0 β MAX.
So: minimum at (3,β22), maximum at (β1,10).
Worked qualitative. Why does the second derivative tell us min vs max?
- Second derivative = rate of change of GRADIENT.
- >0: gradient is INCREASING (going from β to +) β curve curves UP β MIN.
- <0: gradient is DECREASING (+ to β) β curve curves DOWN β MAX.
Edexcel tip. Always determine NATURE β Edexcel asks for it. Use second derivative or sign change test.