Chapter 7 - Section 7.5 - More Trigonometric Equations - 7.5 Exercises - Page 574: 10

$\theta=\frac{3\pi}{4}$, $\theta=\frac{\pi}{4}$

$\cos 2\theta=\cos^2\theta-\frac{1}{2}$ $2\cos^2\theta-1=\cos^2\theta-\frac{1}{2}$ $\cos^2\theta=\frac{1}{2}$ $\cos \theta=\pm\frac{\sqrt{2}}{2}$ If $\cos \theta=\frac{\sqrt{2}}{2}$, then $\theta=\frac{\pi}{4}+k\pi$. If $\cos \theta=-\frac{\sqrt{2}}{2}$, then $\theta=\frac{3\pi}{4}+k\pi$.