Do You Love Summations?

Algebra Level 4

n = 1 101 ( 2 n + 1 1 2 + 2 2 + 3 2 + + n 2 ) \large \displaystyle \sum_{n=1}^{101} \left( \dfrac{2n+1}{1^2 + 2^2 + 3^2 + \cdots + n^2} \right )

If the value of above expression can be written in the form A B \dfrac{A}{B} , where A A and B B are positive coprime integers, find A + B A+B .


The answer is 118.

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1 solution

Akshat Sharda
Feb 26, 2016

Recall that the sum of the first n n perfect squares is 1 2 + 2 2 + 3 2 + + n 2 = 1 6 n ( n + 1 ) ( 2 n + 1 ) 1^2 + 2^2 + 3^2 + \cdots + n^2 = \dfrac16 n(n+1)(2n+1) . (reference: sum of n, n², or n³ ).

= n = 1 101 ( 2 n + 1 i = 1 n i 2 ) = n = 1 101 ( 2 n + 1 n ( n + 1 ) ( 2 n + 1 ) 6 ) = 6 n = 1 101 1 n ( n + 1 ) = 6 n = 1 101 ( 1 n 1 n + 1 ) = 6 ( n = 1 101 1 n n = 2 102 1 n ) = 6 ( 1 1 102 ) = 101 17 101 + 17 = 118 \begin{aligned} & = \displaystyle \sum_{n=1}^{101} \left( \frac{2n+1}{\displaystyle \sum_{i=1}^{n} i^{2}} \right ) \\ & = \displaystyle \sum_{n=1}^{101} \left( \frac{2n+1}{\frac{n(n+1)(2n+1)}{6}} \right ) \\ & = 6 \displaystyle \sum_{n=1}^{101} \frac{1}{n(n+1)} \\ & = 6 \displaystyle \sum_{n=1}^{101}\left( \frac{1}{n}-\frac{1}{n+1}\right) \\ & = 6\left( \displaystyle \sum_{n=1}^{101}\frac{1}{n}- \displaystyle \sum_{n=2}^{102} \frac{1}{n} \right) \\ & = 6\left(1-\frac{1}{102}\right) \\ & = \frac{101}{17} \\ & \Rightarrow 101+17 =\boxed{118} \end{aligned}

Awawawa... I answered 708... I forgot to simplify 6 101 102 6 \cdot \frac{101}{102} ...

Manuel Kahayon - 5 years, 3 months ago

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Same mistake here. Lol

Agil Saelan - 5 years, 3 months ago

It may be a pretty stupid question so please bare with me. In the 6th step, how and why were the summation simplified to 1 and 1/102? Thanks

Mihir Chaturvedi - 5 years, 3 months ago

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n = 1 101 ( 1 n 1 n + 1 ) n = 1 101 1 n n = 2 102 1 n 1 + 1 2 + 1 3 + + 1 101 1 2 1 3 1 101 1 102 1 1 102 = 101 102 \displaystyle \sum_{n=1}^{101}\left( \frac{1}{n}-\frac{1}{n+1}\right) \\ \displaystyle \sum_{n=1}^{101}\frac{1}{n}-\displaystyle \sum_{n=2}^{102} \frac{1}{n} \\ 1+\frac{1}{2}+\frac{1}{3}+\ldots +\frac{1}{101}-\frac{1}{2}-\frac{1}{3}-\ldots -\frac{1}{101}-\frac{1}{102} \\ 1-\frac{1}{102}=\frac{101}{102}

Akshat Sharda - 5 years, 3 months ago

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Thanks a lot! That was a quick reply!

Mihir Chaturvedi - 5 years, 3 months ago

Check out the linked wiki!

Calvin Lin Staff - 5 years, 3 months ago

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Why is editing done to my solution?

Akshat Sharda - 5 years, 3 months ago

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@Akshat Sharda We edit some solutions to make them more presentable and easy to read.

This includes converting equations into Latex so that others can understand them at a glance, and adding explanations / justifications for steps that were taken.

Calvin Lin Staff - 5 years, 3 months ago

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@Calvin Lin Alright! If thats good for the community! :-)

Akshat Sharda - 5 years, 3 months ago

Very overrated problem.

Rohit Udaiwal - 5 years, 3 months ago

Same way buddy

Kaustubh Miglani - 5 years, 3 months ago

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