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trigonometric identities are equalities that involve trigonometric functions that are true for every single value
of the occurring variables. Geometrically, these are identities involving certain functions of one or more angles.
These are distinct from triangle identities, which are identities involving both angles and side lengths of a
triangle. Only the former are covered in this article.
These identities are useful whenever expressions
involving trigonometric functions need to be simplified. An important application is the integration of
non-trigonometric functions: a common technique involves first using the substitution rule with a trigonometric
function, and then simplifying the resulting integral with a trigonometric identity. - Wikipedia