Perpendicular line to tangent

  • Thread starter Willowz
  • Start date
  • #1
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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?
 
Last edited:

Answers and Replies

  • #2
I like Serena
Homework Helper
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Hi Willowz! :smile:

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.
 

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