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Homework Help: Derivatives of Trigonometric Functions

  1. Jun 3, 2008 #1
    [SOLVED] Derivatives of Trigonometric Functions

    I need to find the critical numbers of this function:
    y = cos x - sin x where -pi <= x <= pi

    I found the derivative as:

    dy/dx = -(sin x + cos x)

    But when I equate dy/dx to zero, I get:

    sin x + cos x = 0....where do I go from here?
  2. jcsd
  3. Jun 3, 2008 #2


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    Well when does sinx = -cosx?
  4. Jun 3, 2008 #3
    Ha, lolz I can't believe I didn't think of tan x = -1. (It's getting late, and I've only had 5 hours of sleep these past two nights.) Thanks for the help. =D
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