How do you use the unit circle to prove the double angle formulas for sine and cosine?
Using the unit circle to prove the double angle formulas for sine and cosine?
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trigonometry
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0I'd suggest deriving the formula for sums of angles. – 2012-04-25
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0Use complex numbers. – 2012-04-25
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0I 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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1@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