#An infinite series

Algebra Level 4

x = 1 x x 4 + 4 = p q \sum_{x=1}^{\infty}\frac{x}{x^4+4}=\frac{p}{q} Here gcd ( p , q ) = 2 \gcd(p,q)=2 . Enter p q |p-q| .


The answer is 10.

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2 solutions

Chew-Seong Cheong
May 13, 2021

S = x = 1 x x 4 + 4 By Sophie Germain identity = x = 1 x ( ( x + 1 ) 2 + 1 2 ) ( ( x 1 ) 2 + 1 2 ) = x = 1 x ( x 2 + 2 x + 2 ) ( x 2 2 x + 2 ) = x = 1 1 4 ( 1 x 2 2 x + 2 1 x 2 + 2 x + 2 ) = 1 4 x = 1 ( 1 ( x 1 ) 2 + 1 1 ( x + 1 ) 2 + 1 ) = 1 4 ( 1 1 1 5 + 1 2 1 10 + 1 5 1 17 + 1 10 1 26 + ) = 1 4 ( 1 + 1 2 ) = 3 8 = 6 16 \begin{aligned} S & = \sum_{x=1}^\infty \frac x \blue {x^4+4} & \small \blue{\text{By Sophie Germain identity}} \\ & = \sum_{x=1}^\infty \frac x{((x+1)^2 + 1^2)((x-1)^2+1^2)} \\ & = \sum_{x=1}^\infty \frac x{(x^2 + 2x + 2)(x^2 - 2x + 2)} \\ & = \sum_{x=1}^\infty \frac 14 \left(\frac 1{x^2 - 2x + 2} - \frac 1{x^2 + 2x + 2} \right) \\ & = \frac 14 \sum_{x=1}^\infty \left(\frac 1{(x-1)^2+1} - \frac 1{(x+1)^2 + 1} \right) \\ & = \frac 14 \left(\frac 11 - \blue{\cancel{\frac 15}} + \frac 12 - \blue{\cancel{\frac 1{10}}} + \red {\cancel{\frac 15}} - \blue{\cancel{\frac 1{17}}} + \red{\cancel{\frac 1{10}}} - \blue{\cancel{\frac 1{26}}} + \cdots \right) \\ & = \frac 14 \left(1 + \frac 12\right) = \frac 38 = \frac 6{16} \end{aligned}

Therefore p q = 6 16 = 10 |p-q| = |6-16| = \boxed{10} .


Reference: Sophie Germain Identity

Rafsan Rcc
May 12, 2021

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