Windstream
Ojk's Precalculus Study Page: Sum and Difference Formulas for Sine and Cosine
An introduction to trigonometric identities, beginning with the sum and difference formulas. Some of the formulas are proven, some are left to the reader. Includes several examples and pictures.
CK-12 Foundation
Ck 12: Trigonometry: Applications of Sum and Difference Formulas
[Free Registration/Login may be required to access all resource tools.] Solving equations and verifying identities with the sum and difference formulas.