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Find the value of
\cos\left((\pi)/(9)\right)+\cos\left((3\pi)/(9)\right)+\cos\left((5\pi)/(9)\right)+\cos\left((7\pi)/(9)\right)

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Recall the identity,


\cos\theta+\cos3\theta+\cos5\theta+\cos7\theta=(\sin8\theta)/(2\sin\theta)

(link to proof in the comments)

so that the required sum has a value of


\cos\frac\pi9+\cos\frac{3\pi}9+\cos\frac{5\pi}9+\cos\frac{7\pi}9=\frac{\sin\frac{8\pi}9}{2\sin\frac\pi9}

Recall another identity,


\sin(\pi-x)=\sin x

which means


\sin\frac{8\pi}9=\sin\left(\pi-\frac\pi9\right)=\sin\frac\pi9

Then the sum's value reduces to 1/2.

User Renetta
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