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

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Using a similar method that he used to prove the sum of angles formula for sine, Sal shows the proof of the cosine sum of angles formula, cos (a+ b) = cos (a) cos (b) - sin (a) sin (b). This proof also uses the SOH-CAH-TOA model and some interesting geometric properties of parallel lines and angles.

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