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Homework Help: Solve this equation please

  1. Oct 27, 2006 #1
    I am doing this problem and I am getting stuck at solving the equation

    problem: Use the second derivative test to determine the concavity of the following function. y = x(cosx) at x = pi/3

    solution: y' = -xsinx + cosx
    y'' = -xcosx - 2sinx = 0

    and then i did
    -xcosx = 2sinx ( i dont know if this is correct)
    and then I am stuck... i dont know how to proceed.

    please help
  2. jcsd
  3. Oct 27, 2006 #2
    The function is upward concave is f''(x) > 0, and downward concave if f''(x) < 0. So just plug x=pi/3 into your function for y''.
  4. Oct 28, 2006 #3
    ohhhh!! duhhh!! I shouldve known... thanks
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