Proof of the trig identity: cos(a+b) = (cos a)(cos b)-(sin a)(sin b).
Videos on trigonometry. Watch the "Geometry" playlist first if you have trouble understanding the topics covered here.
Khan Academy is dedicated to the goal of providing a free world-class education for everyone. Founded by Salman Amin Khan, the website features over 3,200 short-form videos that provide a basic introduction to a variety of topics that span from mathematics to current world events. The site is visited by over four million learners each month, and is widely considered an invaluable learning tool for individuals looking for a quick refresher on a subject. The videos are produced in a conversational style and specially designed to be easily viewed on a computer screen.
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