Proofs of the cosine sum and difference identities.

1. Sum and Difference Proofs
(continued ...)
cosbmw by marymactavish

2. Proofs of the Sum and Difference Identities ...
Rotate ∠ROP so that OP
O lies on the positive x-axis O

3. Recall the Even and Odd Identities
The sine and tangent functions are ODD functions.
Cosine is an EVEN function.
sin(-x) = -sin(x) cos(-x) = cos(x) tan(-x) = -tan(x)

4. Proofs of the Sum and Difference Identities ...
Rotate ∠ROP so that OP
O lies on the positive x-axis O