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Proof: cos(a+b) = (cos a)(cos b)-(sin a)(sin b) https://www.youtube.com/watch?v=V3-xCPDzQ1Q -------------------------------- (يتناول هذا الفيديو اثباتاً لقانون جيب ( أ+ب ----------------------------- شكر خاص لمؤسسة شركاء في التنمية المستدامة -- فلسطين http://psdpal.org