The trigonometric triple-angle identities give a relationship between the basic trigonometric functions applied to three times an angle in terms of trigonometric functions of the angle itself.
The triple angle identity of can be proved in a very similar manner.
From these formulas, we also have the following identities for and in terms of lower powers:
To remember the cosine formula, the trick that I like to use is to read cosine as "dollar." Then, we say
"Dollar thirty is equal to four dollar thirty minus three dollar."
Just leaving a mark here,