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How do you use the unit circle to prove the double angle formulas for sine and cosine?

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    I'd suggest deriving the formula for sums of angles.2012-04-25
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    Use complex numbers.2012-04-25
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    I once saw a direct geometric proof of $\cos\,u=2\cos^2 u-1$, but I don't remember where I saw it...2012-04-25
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    @J.M. [Here](http://math.stackexchange.com/questions/126894/how-to-derive-a-double-angle-identity/126898#126898) is one geometric proof of $\cos 2\theta=1-2\sin^2\theta$.2012-04-25

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