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

Learn more: http://www.khanacademy.org/video?v=zw0waJCEc-w
Proof of the trig identity sin(a+b) = (cos a)(sin b) + (sin a)(cos b)

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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)

Length: 8 minutes

Trig identies part 3 (part 5 if you watch the proofs)