Summation Formula: A closed-form expression used to compute the sum of sequences, particularly those involving trigonometric functions, without recourse to term-by-term addition.

Trigonometry Formulas: Check here the list of Trigonometry Formulas for class 10, 11 and 12, ... Some additional formulas for sum and product of angles: cos(x+y) cos(x–y) ...

This uses the trigonometry sum of angles formula and breaks the cosine into a function of sines and cosine with angles that we know the answer for (45° and 30°).

In a new peer-reviewed study, Ne'Kiya Jackson and Calcea Johnson outlined 10 ways to solve the Pythagorean theorem using trigonometry, including a proof they discovered in high school.