The problem asks to prove that $16 \sin(\frac{\pi}{24}) \sin(\frac{5\pi}{24}) \sin(\frac{7\pi}{24}) \sin(\frac{11\pi}{24}) = 1$.

TrigonometryTrigonometric IdentitiesTrigonometric FunctionsProduct-to-Sum FormulasAngle Sum and Difference Identities
2025/4/14

1. Problem Description

The problem asks to prove that 16sin(π24)sin(5π24)sin(7π24)sin(11π24)=116 \sin(\frac{\pi}{24}) \sin(\frac{5\pi}{24}) \sin(\frac{7\pi}{24}) \sin(\frac{11\pi}{24}) = 1.

2. Solution Steps

We need to prove that 16sin(π24)sin(5π24)sin(7π24)sin(11π24)=116 \sin(\frac{\pi}{24}) \sin(\frac{5\pi}{24}) \sin(\frac{7\pi}{24}) \sin(\frac{11\pi}{24}) = 1.
First, we can rewrite sin(11π24)\sin(\frac{11\pi}{24}) as sin(π13π24)=sin(13π24)\sin(\pi - \frac{13\pi}{24}) = \sin(\frac{13\pi}{24}). Also, sin(13π24)=sin(π2+π24)=cos(π24)\sin(\frac{13\pi}{24}) = \sin(\frac{\pi}{2} + \frac{\pi}{24}) = \cos(\frac{\pi}{24}).
So, the expression becomes:
16sin(π24)sin(5π24)sin(7π24)cos(π24)16 \sin(\frac{\pi}{24}) \sin(\frac{5\pi}{24}) \sin(\frac{7\pi}{24}) \cos(\frac{\pi}{24})
=16sin(π24)cos(π24)sin(5π24)sin(7π24)= 16 \sin(\frac{\pi}{24}) \cos(\frac{\pi}{24}) \sin(\frac{5\pi}{24}) \sin(\frac{7\pi}{24})
=8(2sin(π24)cos(π24))sin(5π24)sin(7π24)= 8 (2 \sin(\frac{\pi}{24}) \cos(\frac{\pi}{24})) \sin(\frac{5\pi}{24}) \sin(\frac{7\pi}{24})
We use the formula 2sinxcosx=sin2x2 \sin x \cos x = \sin 2x. Then:
8sin(2π24)sin(5π24)sin(7π24)8 \sin(\frac{2\pi}{24}) \sin(\frac{5\pi}{24}) \sin(\frac{7\pi}{24})
=8sin(π12)sin(5π24)sin(7π24)= 8 \sin(\frac{\pi}{12}) \sin(\frac{5\pi}{24}) \sin(\frac{7\pi}{24})
sin(π12)=sin(15)=sin(4530)=sin45cos30cos45sin30=22322212=624\sin(\frac{\pi}{12}) = \sin(15^\circ) = \sin(45^\circ - 30^\circ) = \sin 45^\circ \cos 30^\circ - \cos 45^\circ \sin 30^\circ = \frac{\sqrt{2}}{2} \frac{\sqrt{3}}{2} - \frac{\sqrt{2}}{2} \frac{1}{2} = \frac{\sqrt{6} - \sqrt{2}}{4}
Also, sin(5π24)=sin(37.5)\sin(\frac{5\pi}{24}) = \sin(37.5^\circ) and sin(7π24)=sin(52.5)\sin(\frac{7\pi}{24}) = \sin(52.5^\circ).
Using the product-to-sum formula, we have:
sinasinb=12[cos(ab)cos(a+b)]\sin a \sin b = \frac{1}{2} [\cos(a-b) - \cos(a+b)]
sin(5π24)sin(7π24)=12[cos(2π24)cos(12π24)]=12[cos(π12)cos(π2)]=12cos(π12)\sin(\frac{5\pi}{24}) \sin(\frac{7\pi}{24}) = \frac{1}{2} [\cos(\frac{2\pi}{24}) - \cos(\frac{12\pi}{24})] = \frac{1}{2} [\cos(\frac{\pi}{12}) - \cos(\frac{\pi}{2})] = \frac{1}{2} \cos(\frac{\pi}{12})
Since cos(π12)=cos(15)=6+24\cos(\frac{\pi}{12}) = \cos(15^\circ) = \frac{\sqrt{6} + \sqrt{2}}{4}, we have
sin(5π24)sin(7π24)=12(6+24)=6+28\sin(\frac{5\pi}{24}) \sin(\frac{7\pi}{24}) = \frac{1}{2} (\frac{\sqrt{6} + \sqrt{2}}{4}) = \frac{\sqrt{6} + \sqrt{2}}{8}
Therefore,
8sin(π12)sin(5π24)sin(7π24)=8(624)(6+28)=8(6232)=8(432)=8(18)=18 \sin(\frac{\pi}{12}) \sin(\frac{5\pi}{24}) \sin(\frac{7\pi}{24}) = 8 (\frac{\sqrt{6} - \sqrt{2}}{4}) (\frac{\sqrt{6} + \sqrt{2}}{8}) = 8 (\frac{6-2}{32}) = 8 (\frac{4}{32}) = 8 (\frac{1}{8}) = 1

3. Final Answer

16sin(π24)sin(5π24)sin(7π24)sin(11π24)=116 \sin(\frac{\pi}{24}) \sin(\frac{5\pi}{24}) \sin(\frac{7\pi}{24}) \sin(\frac{11\pi}{24}) = 1

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