# Integration of Cos Squared x

In this tutorial we shall derive the integral of cosine squared x.

The integration is of the form
$I = \int {{{\cos }^2}xdx}$

This integral cannot be evaluated by the direct formula of integration, so using the trigonometric identity of half angle ${\cos ^2}x = \frac{{1 + \cos 2x}}{2}$, we have
$\begin{gathered} I = \int {\left( {\frac{{1 + \cos 2x}}{2}} \right)dx} \\ \Rightarrow I = \frac{1}{2}\int {\left( {1 + \cos 2x} \right)dx} \\ \Rightarrow I = \frac{1}{2}\int {1dx + \frac{1}{2}\int {\cos 2xdx} } \\ \end{gathered}$

Using the integral formula $\int {\cos kxdx = \frac{{\sin kx}}{k} + c}$, we have
$\begin{gathered} \int {{{\cos }^2}x} dx = \frac{1}{2}x + \frac{1}{2}\frac{{\sin 2x}}{2} + c \\ \Rightarrow \int {{{\cos }^2}x} dx = \frac{1}{2}x + \frac{1}{4}\sin 2x + c \\ \end{gathered}$