# Trig identities - What are they?

Trig identities are pretty tough for most people, because 1) there are so many of them, and 2) they’re hard to remember, and 3) it’s tough to recognize when you’re supposed to use them!

But don’t worry, because the video is going to go over all of that. We’re going to look at the full list of trig identities, including reciprocal identities, quotient identities, pythagorean identities, even-odd identities, co-function identities, sum-difference identities, double-angle identities, half-angle identities (also called power-reducing identities), product-to-sum identities, and sum-to-product identities.

The video will be all about the full list of these identities, plus my tips for how to remember them, how to derive them if you can’t remember them, and why they’re important in helping you simplify other trigonometric expressions.

## In this video you'll learn:

0:00 // What are trig identities?
0:23 // How many trigonometric identities are there?
0:45 // What are the trig identities based on?
1:19 // How are trig functions defined by a right triangle?
2:57 // What trig functions are trig identities based on?
6:19 // What are reciprocal identities?
7:01 // How to prove reciprocal identities?
8:43 // How to remember reciprocal identities?
9:27 // What are quotient identities? How to prove quotient identities?
12:13 // What are pythagorean identities? How to prove pythagorean identities?
15:12 // How to remember pythagorean identities?
17:41 // What are even-odd identities?
18:28 // How to remember even-odd identities?
19:42 // Why are even-odd identities useful?
20:54 // What are co-function identities?
22:17 // How to remember co-function identities?
22:59 // How to prove co-function identities?
26:12 // What are sum-difference identities?
28:09 // How to remember sum-difference identities?
32:55 // Why are sum-difference identities useful?
33:39 // What are double-angle identities? What are half-angle identities?
35:02 // What are product-to-sum identities? What are the sum-to-product identities?
36:11 // How to remember product-to-sum identities?
37:59 // Summary