Partial fraction challenge (2)

Algebra Level 4

S = 1 1 4 + 1 2 + 1 + 2 2 4 + 2 2 + 1 + 3 3 4 + 3 2 + 1 + + 2016 201 6 4 + 201 6 2 + 1 S =\frac{1}{1^{4}+1^{2}+1}+\frac{2}{2^{4}+2^{2}+1}+\frac{3}{3^{4}+3^{2}+1}+\dots+\frac{2016}{2016^{4}+2016^{2}+1}

If the sum above S = 2017 p 2017 q + 1 S = \dfrac{2017p}{2017q+1} , where p p and q q are positive integers , find p + q p+q .


The answer is 3024.

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2 solutions

Chew-Seong Cheong
Jun 22, 2016

S = n = 1 2016 n n 4 + n 2 + 1 Note that n 4 + n 2 + 1 = ( n 2 n + 1 ) ( n 2 + n + 1 ) = n = 1 2016 n ( n 2 n + 1 ) ( n 2 + n + 1 ) = 1 2 n = 1 2016 ( 1 n 2 n + 1 1 n 2 + n + 1 ) = 1 2 ( n = 1 2016 1 n 2 n + 1 n = 1 2016 1 n 2 + n + 1 ) Let n = m 1 = 1 2 ( n = 1 2016 1 n 2 n + 1 m = 2 2017 1 ( m 1 ) 2 + ( m 1 ) + 1 ) = 1 2 ( n = 1 2016 1 n 2 n + 1 m = 2 2017 1 m 2 m + 1 ) Putting m = n = 1 2 ( n = 1 2016 1 n 2 n + 1 n = 2 2017 1 n 2 n + 1 ) = 1 2 ( 1 1 ( 2017 ) 2 2017 + 1 ) = 1 2 ( 1 1 ( 2017 ) ( 2016 ) + 1 ) = 1 2 ( ( 2017 ) ( 2016 ) ( 2017 ) ( 2016 ) + 1 ) = ( 2017 ) ( 1008 ) ( 2017 ) ( 2016 ) + 1 \begin{aligned} S & = \sum_{n=1}^{2016} \frac n{\color{#3D99F6}{n^4+n^2+1}} \quad \quad \small \color{#3D99F6}{\text{Note that }n^4+n^2+1=(n^2-n+1)(n^2+n+1)} \\ & = \sum_{n=1}^{2016} \frac n{\color{#3D99F6}{(n^2-n+1)(n^2+n+1)}} \\ & = \frac 12 \sum_{n=1}^{2016} \left( \frac 1{n^2-n+1} - \frac 1{n^2+n+1} \right) \\ & = \frac 12 \left( \sum_{n=1}^{2016} \frac 1{n^2-n+1} - \sum_{\color{#3D99F6}{n=1}}^{2016} \frac 1{\color{#3D99F6}{n}^2+\color{#3D99F6}{n}+1} \right) \quad \quad \small \color{#3D99F6}{\text{Let }n=m-1} \\ & = \frac 12 \left( \sum_{n=1}^{2016} \frac 1{n^2-n+1} - \sum_{\color{#3D99F6}{m=2}}^{\color{#3D99F6}{2017}} \frac 1{\color{#3D99F6}{(m-1)}^2+\color{#3D99F6}{(m-1)}+1} \right) \\ & = \frac 12 \left( \sum_{n=1}^{2016} \frac 1{n^2-n+1} - \sum_{m=2}^{2017} \frac 1{m^2-m+1} \right) \quad \quad \small \color{#3D99F6}{\text{Putting }m=n} \\ & = \frac 12 \left( \sum_{n=1}^{2016} \frac 1{n^2-n+1} - \sum_{n=2}^{2017} \frac 1{n^2-n+1} \right) \\ & = \frac 12 \left( 1 - \frac 1{(2017)^2-2017+1} \right) \\ & = \frac 12 \left( 1 - \frac 1{(2017)(2016)+1} \right) \\ & = \frac 12 \left( \frac {(2017)(2016)}{(2017)(2016)+1} \right) \\ & = \frac {(2017)(1008)}{(2017)(2016)+1} \end{aligned}

p + q = 1008 + 2016 = 3024 \implies p+q = 1008 + 2016 = \boxed{3024}

Tommy Li
Jun 22, 2016

k 4 + k 2 + 1 = ( k 2 k + 1 ) ( k 2 + k + 1 ) k^{4}+k^{2}+1=(k^{2}-k+1)(k^{2}+k+1)

k k 4 + k 2 + 1 = 1 2 ( 1 k 2 k + 1 1 k 2 + k + 1 ) \frac{k}{k^{4}+k^{2}+1}=\frac{1}{2}(\frac{1}{k^{2}-k+1}-\frac{1}{k^{2}+k+1})

S = n = 1 2016 k k 4 + k 2 + 1 S=\displaystyle\sum_{n=1}^{2016}\frac{k}{k^{4}+k^{2}+1}

S = n = 1 2016 1 2 ( 1 k 2 k + 1 1 k 2 + k + 1 ) S=\displaystyle\sum_{n=1}^{2016}\frac{1}{2}(\frac{1}{k^{2}-k+1}-\frac{1}{k^{2}+k+1})

S = 1 2 ( n = 1 2016 1 k 2 k + 1 n = 1 2016 1 k 2 + k + 1 ) S=\frac{1}{2}(\displaystyle\sum_{n=1}^{2016}\frac{1}{k^{2}-k+1}-\displaystyle\sum_{n=1}^{2016}\frac{1}{k^{2}+k+1})

S = 1 2 ( n = 1 2016 1 k 2 k + 1 n = 2 2017 1 k 2 k + 1 ) S=\frac{1}{2}(\displaystyle\sum_{n=1}^{2016}\frac{1}{k^{2}-k+1}-\displaystyle\sum_{n=2}^{2017}\frac{1}{k^{2}-k+1})

S = 1 2 ( 1 2017 201 7 2 2017 + 1 ) S=\frac{1}{2}(1-\frac{2017}{2017^{2}-2017+1})

S = 1 2 ( 2017 ( 2016 ) 2017 ( 2016 ) + 1 ) S=\frac{1}{2}(\frac{2017(2016)}{2017(2016)+1})

S = 2017 ( 1008 ) 2017 ( 2016 ) + 1 S=\frac{2017(1008)}{2017(2016)+1}

p + q = 1008 + 2016 = 3024 \Rightarrow p+q=1008+2016=3024

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