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#1
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µÃÕ⡳ sin(pi/14)sin(3pi/14)sin(5pi/14)
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$sin(\frac{\pi}{14})\times sin(\frac{3\pi}{14})\times sin(\frac{5\pi}{14})$
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#2
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$\sin (\pi/14) = \sin[(7\pi - 6\pi)/14] = \cos (6\pi/14) = -\cos(4\pi/7)$
$\sin (3\pi/14) = \cos(2\pi/7)$ $\sin(5\pi/14) = \cos (\pi/7) = -\cos(8\pi/7)$ ´Ñ§¹Ñé¹ â¨·Âì = $\cos A \cos 2A \cos 4A $ àÁ×èÍ $A = 2\pi/7$ $\cos A \cos 2A \cos 4A = \frac{2\sin A}{2\sin A}(\cos A \cos 2A \cos 4A) = \frac{\sin 2A \cos 2A \cos 4A}{2\sin A} = ... = \frac{\sin 8A}{8\sin A} = \frac{\sin (16\pi/7)}{8\sin(\pi/7)} = \frac{1}{8}$
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#3
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ÍéÒ§ÍÔ§:
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