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I am trying to understand why $$\sin(4\pi/5) = \sin(\pi/5)$$Is there any formula that could help me? Thank you

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1 Answer

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By the identity $\sin(\pi-x) = \sin x$, we have,

$\sin(\frac{\pi}{5}) = \sin(\pi - \frac{\pi}{5})= \sin\frac{4\pi}{5}$

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