### Home > PC > Chapter 11 > Lesson 11.2.1 > Problem11-75

11-75.

Simplify the following into a single trigonometric term.

1.  $\cos a \cos b -\sin a \sin b$

Review the angle sum identity for cosine.

1. $20 \sin(3x)\cos(3x)$

Review the double-angle identity for sine.

1. $\sin(3x)\cos(x)-\cos(3x)\sin(x)$

Review the angle difference identity for sine.

1. $\sin^2(4y)-\cos^2(4y)$

This expression can be rewritten as $−(\cos^2(4y) − \sin^2(4y))$.