Tangent line at (a,f(a)):
yβf(a)=fβ²(a)(xβa).
Normal line (perpendicular to tangent) at the same point:
yβf(a)=βfβ²(a)1β(xβa)(providedΒ fβ²(a)ξ =0).
Worked example. Find the equations of the tangent and normal to y=x3β4x+5 at x=1.
y(1)=1β4+5=2. Point: (1,2).
yβ²=3x2β4. yβ²(1)=β1. Gradient of tangent = β1.
Tangent: yβ2=β1(xβ1)βy=βx+3.
Normal: gradient =1 (negative reciprocal of β1).
Normal: yβ2=1(xβ1)βy=x+1.
Strategy (do every time):
- Find y-coordinate of the point.
- Differentiate.
- Evaluate fβ²(a) at the x-value.
- Write tangent equation in point-gradient form.
- Take negative reciprocal for the normal.
This is the bread-and-butter of AA SL calculus β appears in every Paper 1 and Paper 2.