TrigExpand

TrigExpand[expr] expands out trigonometric functions in expr.

Examples

Expand a double angle:

TrigExpand[Sin[2x]]
(* 2 Cos[x] Sin[x] *)

Expand a sum:

TrigExpand[Cos[a + b]]
(* Cos[a] Cos[b] - Sin[a] Sin[b] *)

Please visit the official Wolfram Language Reference for more details.