The basic sum-to-product identities for sine and cosine are as follows:
From these identities, we can also infer the difference-to-product identities:
and the tangent sum and difference-to-product identities:
The proof of the basic sum-to-product identity for sine proceeds as follows:
The proofs for cosine and tangent are similar.
In triangle , prove that
Find all solutions in the domain to the equation
Hence, the solutions are .
Write as a product of trigonometric functions.