Problem_
Find the value of $$\sum_{n=1}^\infty\frac{n^2}{(n+1)(n+2)(n+3)(n+4)}$$
It seems like I have to use the partial sum in order to get the exact value. But making it into the partial fractions was not that easy to me because of the numerator.
I tried to segregate the fraction by putting $A, B, C$ and $D$ as: $$\frac{n^2}{(n+1)(n+2)(n+3)(n+4)}={A\over n+1}+{B\over n+2}+{C\over n+3}+{D\over n+4}$$ By multiplying both hands with $(n+1)(n+2)(n+3)(n+4)$, I could get the values which are $A=1/6, B=-2, C=9/2$ and $D=-8/3$. Unfortunately, there were no significant properties among them.
I've also thought of dividing the numerator into two or more terms such as $$n^2=n(n+1) - n$$ However, this method also did not give me any hint to solve. Are there other ways to evaluate the series looks like that? Thank you for your interest.
First add and subtract 1 to the numerator
it becomes $\sum_{n=1}^\infty \frac{(n+4-5)}{(n+2)(n+3)(n+4)} + \sum_{n=1}^\infty\frac{1}{(n+1)(n+2)(n+3)(n+4)}$
$=\sum_{n=1}^\infty\frac{1}{(n+2)(n+3)} -\sum_{n=1}^\infty\frac{5}{(n+2)(n+3)(n+4)}+\sum_{n=1}^\infty\frac{1}{(n+1)(n+2)(n+3)(n+4)}$.(try evaluating this yourself)
$=\frac{1}{3}-\frac{5}{2} \times \frac{1}{12} + \frac{1}{3} \times \frac{1}{24} = \boxed{\frac{5}{36}}$