Almost halfway through 2017!

Algebra Level 3

( 1 1 2 2 ) ( 1 1 3 2 ) ( 1 1 4 2 ) ( 1 1 201 7 2 ) \left(1 - \frac {1}{2^2}\right)\left(1 - \frac {1}{3^2}\right) \left(1 - \frac {1}{4^2}\right)\cdots \left(1 - \frac {1}{2017^2}\right)

If the above product can be expressed as a 2 b \displaystyle \frac {a}{2b} , where a a and b b are coprime positive integers, then find a + b \displaystyle a + b .


The answer is 4035.

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1 solution

Zach Abueg
May 16, 2017

Relevant wiki: Telescoping Series - Product

1 1 k 2 = k 2 1 k 2 = ( k 1 ) ( k + 1 ) k 2 \displaystyle 1 - \frac{1}{k^2} = \frac{k^2 - 1}{k^2} = \frac{(k - 1)(k + 1)}{k^2}

Thus,

( 1 1 2 2 ) ( 1 1 3 2 ) ( 1 1 201 7 2 ) = k = 2 2017 ( k 1 ) ( k + 1 ) k 2 \displaystyle \Bigg(1 - \frac{1}{2^2}\Bigg)\Bigg(1 - \frac{1}{3^2}\Bigg) \cdots \Bigg(1 - \frac{1}{2017^2}\Bigg) = \prod_{k \ = \ 2}^{2017} \frac{(k - 1)(k + 1)}{k^2}

1 3 2 2 2 4 3 2 3 5 4 2 2015 2017 201 6 2 2016 2018 201 7 2 = 2018 2 2017 \displaystyle \frac{1 \cdot 3}{2^2} \cdot \frac{2 \cdot 4}{3^2} \cdot \frac {3 \cdot 5}{4^2} \cdots \cdots \frac{2015 \cdot 2017}{2016^2} \cdot \frac{2016 \cdot 2018}{2017^2} = \frac{2018}{2 \cdot 2017}

Did you mean k = 2 2017 \prod_{k=2}^{2017}

Richard Costen - 4 years ago

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Yes! Thank you for spotting :)

Zach Abueg - 4 years ago

Hey, this problem is similar like mine. And I post it earlier than you.. See this problem

Fidel Simanjuntak - 3 years, 11 months ago

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Aw man, sorry about that! I hadn't seen that problem of yours before. I just happened to think of it, and stopped at 2017 2017 because that was the year!

Zach Abueg - 3 years, 11 months ago

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No worries man.. It doesn't even matter, this can be happen anytime..

Fidel Simanjuntak - 3 years, 11 months ago

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