Multiplying First Quadrant Cosines: 2016 Of Them

Geometry Level 5

n = 1 2016 cos ( n π 2017 ) \prod \limits^{2016}_{n=1}\cos\left(\frac{n\pi }{2017}\right)

If the product above is equal to A B A^{-B} , where A A and B B are positive integers with A A is a prime number, find A + B A + B .


The answer is 2018.

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2 solutions

Mateus Gomes
Feb 11, 2016

x = n = 1 2016 cos ( n π 2017 ) x = \displaystyle \prod \limits^{2016}_{n=1}\cos \left( \frac{n\pi }{2017} \right) y = n = 1 2016 sin ( n π 2017 ) y = \displaystyle\prod \limits^{2016}_{n=1}\sin\left( \frac{n\pi }{2017} \right) x . y = n = 1 2016 sin ( n π 2017 ) cos ( n π 2017 ) x. y =\displaystyle\prod \limits^{2016}_{n=1}\sin\left( \frac{n\pi }{2017} \right) \cos \left( \frac{n\pi }{2017} \right) x . y = n = 1 2016 1 2 sin ( 2 n π 2017 ) = ( 1 2 ) 2016 [ sin ( 2 π 2017 ) sin ( 4 π 2017 ) sin ( 14 π 2017 ) sin ( 16 π 2017 ) sin ( 18 π 2017 ) sin ( 4032 π 2017 ) ] x. y = \displaystyle\prod_{n=1}^{2016}\frac{1}{2}\sin\left( \frac{2n\pi}{2017} \right) = \bigg(\frac{1}{2}\bigg)^{2016}[\sin \left( \frac{2\pi }{2017} \right) \sin \left( \frac{4\pi }{2017} \right) \cdots \sin \left( \frac{14\pi }{2017} \right) \sin \left( \frac{16\pi }{2017} \right) \sin \left( \frac{18\pi }{2017} \right) \cdots \sin \left( \frac{4032\pi }{2017} \right)] θ + α = 2 π sin ( θ ) = sin ( 2 π α ) \color{#3D99F6}{\boxed{\color{#D61F06}{\boxed{\theta+\alpha=2\pi \rightarrow \sin(\theta)=-\sin(2\pi-\alpha)}}}} x . y = n = 1 2016 1 2 sin ( 2 n π 2017 ) = ( 1 2 ) 2016 × ( 1 ) 1008 [ n = 1 2016 sin ( n π 2017 ) ] x. y = \displaystyle\prod_{n=1}^{2016}\frac{1}{2}\sin\left( \frac{2n\pi}{2017} \right) = \bigg(\frac{1}{2}\bigg)^{2016}\times(-1)^{1008}[\displaystyle\prod \limits^{2016}_{n=1}\sin\left( \frac{n\pi }{2017} \right) ] x . y = ( 1 2 ) 2016 × ( 1 ) 1008 . y x. y =\left( \frac{1}{2} \right) ^{2016}\times\left( -1\right) ^{1008}. y x = ( 1 2 2016 ) = ( 1 A B ) x =\left( \frac{1}{2^{2016}} \right)=(\frac{1}{A^B}) A + B = 2018 \Large\color{#3D99F6}{\boxed{\color{forestgreen}{\boxed{A+B=2018}}}}

N I C E ! ! \huge\color{#0C6AC7}{\mathcal{NICE!!}}

Rishabh Jain - 5 years, 4 months ago
Refaat M. Sayed
Feb 12, 2016

simply we can use the identity P = k = 1 n 1 cos ( k π n ) = sin ( π n / 2 ) 2 n 1 P=\large \prod_{k=1}^{n-1} \cos\left(\frac{k\pi}{n}\right) = \frac{\sin(\pi n/2)}{2^{n-1}} now put n = 2017 n=2017 to get P = 1 2 2016 P=\frac {1}{2^{2016}} So A + B = 2018 \Large\color{#624F41}{\boxed{A+B=2018}}

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