Finding an Equation for a Perpendicular Line to a Tangent at a Given Point

[Willowz]

Homework Statement

Find an equation for the line perpendicular to the tangent to the curve y=x^3-4x+1 at the point (2,1).

Homework Equations

point slope form, m * (-1/m) = -1

The Attempt at a Solution

Derivate: y+3x^2-4
m=8 at x=2
Tangent line: y=8x-15
Answer: y=-x/8-15

Answer in book, y=-x/8 + 5/4

Where'd I go wrong?

 

[I like Serena]

Hi Willowz!

Your line has to contain the point (2,1).
For the perpendicular line this means you get another y-intercept than the tangent line has.