Help us caption and translate this video on Amara.org: http://www.amara.org/en/v/B2va/ Esercizi: http://www.khanacademy.org/exercise/trig_identities_1 Traduzione in Italiano del video Trig identities part 2 (parr 4 if you watch the proofs) http://www.youtube.com/watch?feature=player_embedded&v=ZWSoyUxAQW0 http://www.khanacademy.org/video/trig-identities-part-2--parr-4-if-you-watch-the-proofs?playlist=Trigonometry