trigonometric identities

• How to Calculate Compound Angles in Trigonometry
  Use compound-angle identities to find sine, cosine, or tangent of sums and differences, with sign rules and a worked exact-value example.
• How to Write a Trigonometric Expression in Sine and Cosine
  Replace tangent, cotangent, secant, and cosecant with sine-and-cosine quotients, then simplify; the exact expression is needed for a unique answer.