Learn more: http://www.khanacademy.org/video?v=OLzXqIqZZz0
Introduction to trigonometric identities

Length: 9 minutes

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

Length: 9 minutes

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

Length: 9 minutes

Trig identities part 2 (parr 4 if you watch the proofs)