I try to find the question but I didn't

How do you do it? I'm really stuck on this proof. Can someone please explain?

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    $\begingroup$ What definition of cos and sin are you using? $\endgroup$ – evil999man Apr 19 '14 at 10:07

Try geometrically: draw a unitary circle and an angle $\alpha$. Then $\sin\alpha$ and $\cos\alpha$ and the radius will form exactly a right angled triangle, and the formula you're searching for will be exactly the Pitagora Theorem.

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  • $\begingroup$ Pythagorean* theorem? $\endgroup$ – naslundx Apr 19 '14 at 10:15
  • $\begingroup$ Yes. But I'm Italian and I like to call Greek men with the names I've always used (unless it gives confusion). $\endgroup$ – Joe Apr 19 '14 at 10:25
  • $\begingroup$ I understand. But I'm afraid very different names, such as that one, does provide confusion. $\endgroup$ – naslundx Apr 19 '14 at 10:26

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