Help us caption and translate this video on Amara.org: http://www.amara.org/en/v/B2v9/ Esercizi: http://www.khanacademy.org/exercise/trig_identities_1 Traduzione in Italiano del video Proof: cos(a+b) = (cos a)(cos b)-(sin a)(sin b) http://www.youtube.com/watch?feature=player_embedded&v=V3-xCPDzQ1Q http://www.khanacademy.org/video/proof--cos-a-b-----cos-a--cos-b---sin-a--sin-b?playlist=Trigonometry