Sal takes the mystery out of the trigonometric identities by showing how easily they can be derived. He continues with a problem he started in the video �Trig identities part three (part five if you watch the proofs) and proves the trig identities cos (-a) = cos a and sin (-a) = sin (a). He also proves sin(a + _/2) = cos (a)and sin (a) = cos (a - _/2) along with the difference of angle formulas by using the sum of angle formula.

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