New Year Enigma Contd

Geometry Level 3

sin π 14 sin 3 π 14 sin 5 π 14 = ? \large{\sin \dfrac {\pi}{14}\cdot\sin \dfrac {3\pi}{14}\cdot\sin\dfrac {5\pi}{14} ~=~?}


The answer is 0.125.

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2 solutions

Jessica Wang
Jan 1, 2016

Here I shall provide a relatively elementary approach (in comparison to @Otto Bretscher 's!):

sin π 14 sin 3 π 14 sin 5 π 14 \sin\frac{\pi}{14}\cdot \sin\frac{3\pi}{14}\cdot \sin\frac{5\pi}{14} = cos 6 π 14 cos 4 π 14 cos 2 π 14 =\cos\frac{6\pi}{14}\cdot \cos\frac{4\pi}{14}\cdot \cos\frac{2\pi}{14} = cos π 7 cos 2 π 7 cos 3 π 7 =\cos\frac{\pi}{7}\cdot \cos\frac{2\pi}{7}\cdot \cos\frac{3\pi}{7} = cos π 7 cos 2 π 7 cos 4 π 7 =-\cos\frac{\pi}{7}\cdot \cos\frac{2\pi}{7}\cdot \cos\frac{4\pi}{7} = sin π 7 cos π 7 cos 2 π 7 cos 4 π 7 sin π 7 =\frac{-\sin\frac{\pi}{7}\cdot \cos\frac{\pi}{7}\cdot \cos\frac{2\pi}{7}\cdot \cos\frac{4\pi}{7}}{\sin\frac{\pi}{7}} = sin 2 π 7 cos 2 π 7 cos 4 π 7 2 sin π 7 =\frac{-\sin\frac{2\pi}{7}\cdot \cos\frac{2\pi}{7}\cdot \cos\frac{4\pi}{7}}{2\sin\frac{\pi}{7}} = sin 4 π 7 cos 4 π 7 4 sin π 7 =\frac{-\sin\frac{4\pi}{7}\cdot \cos\frac{4\pi}{7}}{4\sin\frac{\pi}{7}} = sin 8 π 7 8 sin π 7 =\frac{-\sin\frac{8\pi}{7}}{8\sin\frac{\pi}{7}} = 1 8 sin π 7 sin π 7 =\frac{1}{8}\cdot \frac{\sin\frac{\pi}{7}}{\sin\frac{\pi}{7}} = 0.125 . =\boxed{0.125}.

Very clever manipulation, nicely done

Lawrence Mayne - 5 years, 5 months ago
Otto Bretscher
Dec 31, 2015

Nice variant of Morrie's Law

Using cos ( x ) = sin ( π / 2 x ) \cos(x)=\sin(\pi/2-x) and k = 0 n 1 cos ( 2 k x ) = sin ( 2 n x ) 2 n sin ( x ) \prod_{k=0}^{n-1}\cos(2^kx)=\frac{\sin(2^nx)}{2^n\sin(x)} we can write the given expression as cos ( 8 π / 14 ) cos ( 4 π / 14 ) cos ( 2 π / 14 ) = sin ( 16 π / 14 ) 2 3 sin ( 2 π / 14 ) = 0.125 -\cos(8\pi/14)\cos(4\pi/14)\cos(2\pi/14)=-\frac{\sin(16\pi/14)}{2^3\sin(2\pi/14)}=\boxed{0.125}

cos ( x ) sin ( x π 2 ) \cos(x) \neq \sin\left(x - \dfrac{\pi}{2}\right)

Akhil Bansal - 5 years, 5 months ago

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oops...typo.. the New Year's festivities are getting me confused

Otto Bretscher - 5 years, 5 months ago

lol.. it's a simple problem just type it in the calculator! Why make the simple complicated. lol. peace man

Okay Fine - 5 years, 5 months ago

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But you won't have calculator during exam..

Akhil Bansal - 5 years, 5 months ago

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Here in the philippines our exams are using the aid of a calculator.. stupid isn't it? We're trained to be idiots and not intellectuals

Okay Fine - 5 years, 5 months ago

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@Okay Fine Filipino here! and what did you say ? idiots? Shame on you, I know you are a filipino too!

Jun Arro Estrella - 5 years, 5 months ago

@Okay Fine I wish i lived in philippines.........

Siddhant Chaudhari - 5 years, 5 months ago

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