Deriving the Sum and Difference Identities for Cosine | Finding cos (s + t)

In this lesson, we will learn about the sum and difference identity for cosine. We will derive both formulas and then look at some typical examples that involve the two formulas.

00:00 Cosine of a sum or difference formulas
01:53 Deriving the formula for the sum identity for cosine
12:05 Deriving the formula for the difference identity for cosine
13:12 Finding exact cosine function values with sum and difference identities Example #1
15:44 Finding exact cosine function values with sum and difference identities Example #2
18:20 Finding exact cosine function values with sum and difference identities Example #3
19:20 Cofunction identities
22:42 Verifying identities with sum and difference identities Example #1
24:14 Verifying identities with sum and difference identities Example #2
25:24 Finding cos(s + t)