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

1. ## 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).

2. 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 $-2\sin x (\cos x + 1) = 0$.