+13 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.
Is my answer right?
