I've been trying to prove the following formula (for n>1 natural, a,b non-zero reals), but I don't know where to start.
n∑j=1(n−1j−1)(a−j+1b−n+1)(ab)j−1(b−ab)n−j=ab
Wolfram says it's right, but so far I've been unable to give a proof. I wonder if there's a combinatorial proof to it. Any help, hint or reference will be much appreciated.
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