# Slippery Sum!

Calculus Level 4

$\displaystyle\sum _{ r=1 }^{ \infty }{ \frac { r\left( r+1 \right) }{ { 3 }^{ r } } }$

If the summation above equals to $$\frac m n$$ for coprime positive integers $$m,n$$, what is the value of $$m+n$$?

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