sequences and series - Using roots of unity to prove that $\cos\frac{\pi }{2n}\cos\frac{2\pi}{2n}\cdots\cos\frac{(n-1)\pi}{2n}=\frac{\sqrt{n}}{2^{n-1}}$ - Mathematics Stack Exchange
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If 0<cosx<pi/2, what is the value of cos (x+pi)? - Quora
Prove the following (i) cos(π/2 + x) = - sinx (ii) sin(π/2 + x) = cosx - Sarthaks eConnect | Largest Online Education Community