8:31 AM
These three questions seem like duplicates to me:
If you have a look at the linked questions to the first one, there is also a fourth question about the same sum. But since it specifically asks for combinatorial proof, I would not consider it to be a duplicate.
Since one of those questions was posted by me, it is probably better if I let other users decide whether they should be closed and in which direction.
(At least in situations where the questions already have several answers and choice of the direction is not clear-cut, this seems to me the best way to go. Although I admit that it might seem that I am dumping work on others.)
