# Derivative of Cosine Squared Function

In this tutorial we shall discuss derivative of cosine squared function and its related examples. It can be proved by definition of differentiation.
We have function of the form

By the definition of differentiation we have

Putting the value of function in equation (i), we get

Using formula from trigonometry

Example: Find the derivative of

We have the given function as

Differentiation with respect to variable $x$, we get

Using the rule, $\frac{d}{{dx}}{\cos ^2}x = - 2\cos x\sin x$, we get