Chapter 7 - Section 7.3 - Double-Angle, Half-Angle, and Product-Sum Formulas - 7.3 Exercises - Page 561: 64

$2\cos\frac{11x}{2}\cos\frac{7x}{2}$

Use the third Sum-to-Product Formula, $\cos x+\cos y=2\cos\frac{x+y}{2}\cos\frac{x-y}{2}$. $\cos 9x+\cos 2x$ $=2\cos\frac{9x+2x}{2}\cos\frac{9x-2x}{2}$ $=2\cos\frac{11x}{2}\cos\frac{7x}{2}$