Tính $A=cos\frac{\pi}{2^2}.cos\frac{\pi}{2^3}.cos\frac{\pi}{2^4}.....cos\frac{\pi}{2^{n+1}}$
Ta có $2^n.Sin \frac{\pi}{2^{n+1}} .A = 2^{n-1}.cos\frac{\pi}{2^2}...cos\frac{\pi}{2^n}.Sin\frac{\pi}{2^n} = cos\frac{\pi}{2} = 0 \Rightarrow A = ...$
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