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Homework Help: Finding the X-coordinate of a normal to the tangent

  1. Apr 5, 2013 #1
    1. The problem statement, all variables and given/known data

    Tangent to C at point P(2,7) has an equation of y=3x+1.

    Point Q also lies on C and is perpendicular to the tangent,
    show that the X-coordinate is [1/3(2+√6)]

    2. Relevant equations
    curve C has equation y= x3-2x2-x+9
    dy/dx = 3x2-4x-1

    3. The attempt at a solution
    gradient of normal has to = -1/3, right? considering that the tangent that is is perpendicular to is +3

    I'm not sure as, the point that the normal is passing through hasn't been specified.

    Here is the actual question: http://i259.photobucket.com/albums/hh299/the-real-guitar-hero/CapturePNGnnn_zpsd170ce09.png

    Part C.
    Last edited: Apr 5, 2013
  2. jcsd
  3. Apr 5, 2013 #2


    Staff: Mentor

    Problems involving derivatives should be posted in the Calculus & Beyond section. I am moving this thread to that section.
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