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More tangent lines

  1. Oct 16, 2007 #1
    1. The problem statement, all variables and given/known data

    y = 9sin(x)cos(x)

    Find all points where tangent line is horizontal.

    3. The attempt at a solution

    I get y' = 9cos^2x - 9sin^2X

    I plug in zero for the slope and get 9 but I'm stumped after that. How can I get all the horizontal tangent line points?
    Last edited: Oct 16, 2007
  2. jcsd
  3. Oct 17, 2007 #2


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    Staff Emeritus
    Science Advisor

    You do exactly what you say to do; that is, find the points where the derivative of the graph is zero. It boils down to solving cos^2x-sin^2x=0. Can you do this?

    [Hint: cos^2x=1-sin^2x]
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