Helping understand generating functions

Question

I have a problem with understanding following example: We are given $a_n$ - number of ways that to put $n$ balls(indistinguishable) into $k$ boxes, $k$ is fixed. Now we calculate $\sum_{n=1}^{\infty}a_n\cdot x^n=(1+x+x^2\ldots)\cdot\ldots\cdot(1+x+x^2+\ldots)=\frac{1}{(1-x)^{k}}$ I don't see how we come up with first equality. I will be very glad for simple explanation.

Answer 1

Note that choosing the number of ways that to put $n$ indistinguishable balls into $k$ boxes, is exactly the same as asking for the number of $k$ non-negative integers that add up to $n$, and is therefore the same as asking what the coefficient is (read "how many") of $x^n$ is ("are") in the expression: $$\prod_{i=1}^k\left(\sum_{j=0}^\infty x^j\right)$$