1. Warm Up
Find each product, if possible.
1. AB
2. BA

2. Objectives
Evaluate trigonometric expressions by using sum and difference identities.
Use matrix multiplication with sum and difference identities to perform rotations.

3. Vocabulary
rotation matrix

4. Matrix multiplication and sum and difference identities are tools to find the coordinates of points rotated about the origin on a plane.

7. Example 1A: Evaluating Expressions with Sum and Difference Identities
Find the exact value of cos 15°.
Write 15° as the difference 45° – 30° because trigonometric values of 45° and 30° are known.
cos 15° = cos (45° – 30°)
Apply the identity for cos (A – B).
= cos 45° cos 30° + sin 45° sin 30°
Evaluate.
Simplify.