# horizontal tangent, set it to zero, now what?

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• Feb 11th 2010, 10:18 PM
BugzLooney
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).
• Feb 11th 2010, 10:19 PM
mr fantastic
Quote:

Originally Posted by BugzLooney
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 $-2\sin x (\cos x + 1) = 0$.