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

http://www.greenemath.com/ 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)