What are the basic trigonometric identities? | Purplemath
https://www.purplemath.com/modules/idents.htm
WEBIn other words, the identities allow you to restate a trig expression in a different format, but one which has the exact same value. There are loads of trigonometric identities, but the following are the ones you're most likely to see and use. Basic & Pythagorean, Angle-Sum & -Difference, Double-Angle, Half-Angle, Sum, Product.
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