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Math Help - horizontal tangent, set it to zero, now what?

  1. #1
    Junior Member BugzLooney's Avatar
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    horizontal tangent, set it to zero, now what?

    Find all points on the graph of the function f(x) = 2cos(x) + cos(x)^2 at which the tangent line is horizontal. I found the derivative to be -sinx(cosx + 2).
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  2. #2
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    mr fantastic's Avatar
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    Quote Originally Posted by BugzLooney View Post
    Find all points on the graph of the function f(x) = 2cos(x) + cos(x)^2 at which the tangent line is horizontal. I found the derivative to be -sinx(cosx + 2). Mr F says: This should be -2sinx(cosx + 1)
    Now solve -2\sin x (\cos x + 1) = 0.
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