CK-12 Foundation
Ck 12: Trigonometry: Product to Sum Formulas for Sine and Cosine
[Free Registration/Login may be required to access all resource tools.] Examples of how to transform a product of trigonometric functions into a sum of trigonometric functions
CK-12 Foundation
Ck 12: Trigonometry: Sum to Product Formulas for Sine and Cosine
[Free Registration/Login may be required to access all resource tools.] Examples of transforming a sum of trigonometric functions to a product of trigonometric functions.