##
The Derivatives of Trigonometric Functions - Exercise 2

**Exercise 2.** Find the *x*-coordinates of all points on the
graph of
in the interval
at which
the tangent line is horizontal.

**Answer.** The points (*x*,*f*(*x*)) at which the tangent line is
horizontal are the ones for which *f*'(*x*) = 0. Let us first find
*f*'(*x*). We have

So we have to solve
which gives
.
We get

**
**

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