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Prove-that-4sin-2-36-1-2-cos72-




Question Number 7187 by 314159 last updated on 15/Aug/16
Prove that ((√(4sin^2 36−1))/2)=cos72.
$${Prove}\:{that}\:\frac{\sqrt{\mathrm{4}{sin}^{\mathrm{2}} \mathrm{36}−\mathrm{1}}}{\mathrm{2}}={cos}\mathrm{72}. \\ $$

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