Evaluate the infinite product P=∏n=1∞cos(θ2n)P = \prod_{n=1}^{\infty} \cos\left(\frac{\theta}{2^n}\right)P=∏n=1∞cos(2nθ).
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sinθ2θ\frac{\sin \theta}{2\theta}2θsinθ
sin(θ/2)θ/2\frac{\sin(\theta/2)}{\theta/2}θ/2sin(θ/2)
sinθθ\frac{\sin \theta}{\theta}θsinθ