Khan Academy on a Stick
Proof: cos(a+b) = (cos a)(cos b)-(sin a)(sin b)
Proof of the trig identity: cos(a+b) = (cos a)(cos b)-(sin a)(sin b)
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Proof of the trig identity: cos(a+b) = (cos a)(cos b)-(sin a)(sin b)
If the above player doesn't work, you can try this direct link.