Welcome 2016! Part 17

Algebra Level 4

S = 2 2 + 1 2 2 1 + 3 2 + 1 3 2 1 + + 201 6 2 + 1 201 6 2 1 \large S= \dfrac{2^2+1}{2^2-1}+\dfrac{3^2+1}{3^2-1}+ \ldots + \dfrac{2016^2+1}{2016^2-1}

If S S denotes the value of the above expression, then find the value of S + 2016 \lfloor S \rfloor + 2016 .


The answer is 4032.

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2 solutions

Chew-Seong Cheong
Dec 25, 2015

S = n = 2 2016 n 2 + 1 n 2 1 = n = 2 2016 n 2 1 + 2 n 2 1 = n = 2 2016 ( 1 + 2 n 2 1 ) = n = 2 2016 ( 1 + 1 n 1 1 n + 1 ) = n = 2 2016 1 + n = 2 2016 1 n 1 n = 2 2016 1 n + 1 = n = 1 2015 1 + n = 1 2015 1 n n = 3 2017 1 n = 2015 + 1 1 + 1 2 1 2016 1 2017 2016.5 S + 2016 = 2016 + 2016 = 4032 \begin{aligned} S & = \sum_{n=2}^{2016} \frac {n^2+1}{n^2 - 1}\\ & = \sum_{n=2}^{2016} \frac {n^2 - 1+2}{n^2 - 1}\\&= \sum_{n=2}^{2016}\left(1+\frac {2}{n^2 - 1}\right) \\ & = \sum_{n=2}^{2016}\left(1+\frac{1}{n-1}-\frac {1}{n+1}\right) \\ & = \sum_{n=2}^{2016} 1+ \sum_{n=2}^{2016} \frac{1}{n-1} - \sum_{n=2}^{2016} \frac {1}{n+1} \\ & = \sum_{n=1}^{2015} 1+ \sum_{n=1}^{2015} \frac{1}{n} - \sum_{n=3}^{2017} \frac {1}{n} \\ &= 2015+\frac{1}{1} +\frac{1}{2} - \frac {1}{2016}-\frac {1}{2017} \approx 2016.5 \\ \Rightarrow \lfloor S \rfloor +2016&=2016+2016= \boxed {4032} \end{aligned}

Can't understand last line.

Mishkat Islam - 5 years, 5 months ago

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I have added lines to the solution. See if it helps.

Chew-Seong Cheong - 5 years, 5 months ago

How did you figure out the fourth line of the solution. I know it's correct but what was the way to figure that out from the third line?

Anupam Nayak - 5 years, 5 months ago

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Through practice. This is a common method used to make the summation telescopic. For example, 1 n ( n + 1 ) = 1 n 1 n + 1 \frac{1}{n(n+1)} = \frac{1}{n} - \frac{1}{n+1} and 1 n ( n + 1 ) ( n + 2 ) = ( 1 n 2 n + 1 + 1 n + 2 ) \frac{1}{n(n+1)(n+2)} = \left(\frac{1}{n}-\frac{2}{n+1}+\frac{1}{n+2}\right) . You may need to do trial and error to find out.

Chew-Seong Cheong - 5 years, 5 months ago

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Is it possible to do this with any expression in the denominator?

Anupam Nayak - 5 years, 5 months ago

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@Anupam Nayak Of course not all.

Chew-Seong Cheong - 5 years, 5 months ago

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@Chew-Seong Cheong Then how do we determine if it is possible or not? Is there a way to check it because if it was not possible for that expression, we will waste a lot of time but with no results.

Anupam Nayak - 5 years, 5 months ago
Akshat Sharda
Dec 24, 2015

S = 2 2 + 1 2 2 1 + 3 2 + 1 3 2 1 + + 201 6 2 + 1 201 6 2 1 = n = 1 2015 ( n + 1 ) 2 + 1 ( n + 1 ) 2 1 = n = 1 2015 ( n + 1 ) 2 + 1 n ( n + 2 ) = n = 1 2015 ( 1 n 1 n + 2 + 1 ) = n = 1 2015 1 n n = 3 2017 1 n + n = 1 2015 1 = 1 + 1 2 + n = 3 2015 1 n n = 3 2017 1 n + n = 1 2015 1 = 1 + 1 2 1 2016 1 2017 + 2015 = 1.499 + 2015 = 2016.499 S + 2016 = 2016 + 2016 = 4032 \begin{aligned} S & = \frac{2^2+1}{2^2-1}+\frac{3^2+1}{3^2-1}+ \ldots + \frac{2016^2+1}{2016^2-1} \\ & = \displaystyle \sum^{2015}_{n=1}\frac{(n+1)^2+1}{(n+1)^2-1} \\ & = \displaystyle \sum^{2015}_{n=1}\frac{(n+1)^2+1}{n(n+2)} \\ & = \displaystyle \sum^{2015}_{n=1}\left(\frac{1}{n}-\frac{1}{n+2}+1\right) \\ & = \displaystyle \sum^{2015}_{n=1}\frac{1}{n}- \displaystyle \sum^{2017}_{n=3}\frac{1}{n}+ \displaystyle \sum^{2015}_{n=1}1 \\ & = 1+\frac{1}{2}+\displaystyle \sum^{2015}_{n=3}\frac{1}{n}- \displaystyle \sum^{2017}_{n=3}\frac{1}{n}+ \displaystyle \sum^{2015}_{n=1}1 \\ & = 1+\frac{1}{2}-\frac{1}{2016}-\frac{1}{2017}+2015 \\ & = 1.499+2015=2016.499 \\ & \Rightarrow \lfloor S \rfloor + 2016 = 2016 +2016=\boxed{4032}\end{aligned}

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