## Precalculus: Mathematics for Calculus, 7th Edition

$\frac{1}{2}(\cos 8x+\cos 2x)$
Use the third Sum-to-Product Formula on page 559, $\cos u\cos v=\frac{1}{2}[\cos(u+v)+\cos(u-v)]$ $\cos 5x\cos 3x$ $=\frac{1}{2}[\cos(5x+3x)+\cos(5x-3x)]$ $=\frac{1}{2}(\cos 8x+\cos 2x)$