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# Indistinguishable balls into distinguishable boxes

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How many ways are there to put 2 indistinguishable balls into n distinguishable boxes?

My attempt:

I solved this problem similar way as stars and bars problem. I treated boxes as bars and balls as stars. So there are total $$\binom{n\ -1\ +\ 2}{2} \ =\ \frac{n( n+1)}{2}$$ ways.