Cosine of the Difference of Two Angles (Proof)

Steve Stein
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Published on Sep 21, 2011

I provide a proof that cos (a-b)=cosacosb+sinasinb. The main idea is to create a triangle whose angle is a difference of two other angles, whose adjacent sides, out of simplicity, are both 1. By using both the distance formula and the law of cosines, we can get an equation where cos(a-b) is present.

In this video I used a "toolbox" summarizing key ideas used in the proof. Hopefully the viewer will find it helpful.

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