Sal proves the sum of angles formula for sine, sin (a+ b) = cos (a) sin (b) + sin (a) cos (b), through a clear but rather non-intuitive proof. The proof is interesting as it not only uses the SOH-CAH-TOA model but requires one to recall some interesting geometric properties of parallel lines and angles. A challenging problem after watching this video would be try to prove the cosine version of this identity, which is shown in another video.

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