## Precalculus (6th Edition) Blitzer

$\frac{\pi }{4}$ is the solution of the given equation.
We know that according to the ratio table of trigonometry, the value of $\cos \frac{\pi }{4}$ is provided as $\frac{\sqrt{2}}{2}$. \begin{align} & \cos x=\frac{\sqrt{2}}{2} \\ & \cos \frac{\pi }{4}=\frac{\sqrt{2}}{2} \\ & \frac{\sqrt{2}}{2}=\frac{\sqrt{2}}{2} \end{align} Thus, $\cos x=\frac{\sqrt{2}}{2}$ is true.