Proof: sin(a+b)=(cos a)(sin b)+(sin a)(cos b)

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 uses the SOH-CAH-TOA model and requires one to recall some interesting geometric properties of parallel lines and angles. A challenging problem after watching this video would be trying to prove the cosine version of this identity, shown in another video.