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Homework Help: Perpendicular line to tangent

  1. Oct 30, 2011 #1
    1. The problem statement, all variables and given/known data
    Find an equation for the line perpendicular to the tangent to the curve y=x^3-4x+1 at the point (2,1).

    2. Relevant equations
    point slope form, m * (-1/m) = -1

    3. 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: Oct 30, 2011
  2. jcsd
  3. Oct 30, 2011 #2

    I like Serena

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    Homework Helper

    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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