Infinite sum converging to 2 How do I compute
$$\sum_{r=1}^{\infty} \frac{8r}{4r^4 +1}$$
Calculating first few terms tells me that the sum converges to 2.
I have also tried squeezing the term. 
 A: Have you considered partial sum formulas? 
Wolframalpha finds:
$\displaystyle \sum_{r=1}^n \dfrac{8r}{4r^4+1} = 2-\dfrac{2}{2n^2+2n+1}$
I'd recommend trying to figure out how it calculated that partial sum. Now, the limit as $n\to \infty$ obviously makes the second term vanish.
A: Hint: use that 
$$4r^4+1=(2r^2)^2+1=((2r^2)^2+4r^2+1)-4r^2=(2r^2+1)^2-(2r)^2=(2r^2+2r+1)(2r^2-2r+1)$$
Now, using simple fractions,
$\dfrac{8r}{4r^4+1}=\dfrac{8r}{(2r^2+2r+1)(2r^2-2r+1)}=\dfrac{2}{2r^2-2r+1}-\dfrac{2}{2r^2+2r+1}=2\left(\dfrac{1}{2r^2-2r+1}-\dfrac{1}{2r^2+2r+1}\right)$.
The idea is calculate $\sum\limits_{r\in\Bbb N}\frac{1}{2r^2\pm 2r+1}$ using the fact that $$\frac{1}{2r^2\pm 2r+1}=\int^1_0 x^{2r^2\pm 2r}dx$$ and interchanging the sum with the integral.
An example is here: Finding the infinite sum of a rational function using integrals
A: Partial fraction expansion gives us$$\begin{align*} & \sum\limits_{r=1}^n\frac {8r}{4r^4+1}=\sum\limits_{r=1}^n\frac 2{2r^2-2r+1}-\sum\limits_{r=1}^n\frac 2{2r^2+2r+1}\\ & =\left[2+\frac 2{5}+\frac 2{13}+\cdots+\frac 2{2n^2-2n+1}\right]-\left[\frac 2{5}+\frac 2{13}+\cdots+\frac 2{2n^2+2n+1}\right]\end{align*}$$Notice how all but the last fraction in the second sum cancels out with the fractions in the first sum. Continuing on indefinitely until the end gives us$$\sum\limits_{r=1}^n\frac {8r}{4r^4+1}=2-\frac 2{2n^2+2n+1}$$As $n\to\infty$, the fraction tends to zero, so your sum equals$$\sum\limits_{r\geq1}\frac {8r}{4r^4+1}=2$$
EDIT: To find the partial fraction decomposition, we first factor the denominator as a product of two quadratics. This can be done by adding and subtracting $4r^2$ so the quartic factors as$$4r^4+1=(2r^2+2r+1)(2r^2-2r+1)$$Now, the decomposition is set up as$$\frac {8r}{(2r^2+2r+1)(2r^2-2r+1)}=\frac {Ar+B}{2r^2-2r+1}+\frac {Cr+D}{2r^2+2r+1}$$
