# Cube over power of three sum!

Calculus Level pending

The value of $\sum_{i=1}^n \frac{i^3}{3^i}$ When $$n$$ approaches infinity can be expressed as an improper fraction $$\frac{a}{b}$$ while $$a$$ and $$b$$ are positive integers which are relatively prime. Find $$a+b$$.

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