multinomial

find the coeffcient of (x^a)(y^b) in the expansion (x+y+xy+2)^(2008),
where both a and b are even nos?

2 Answers

nobody tried this?

\sum_{r=1}^{n}{^{n}C_{r}\times ^{r}C_{a}\times ^{r}C_{b}} \;\; ,\; \;a,b \leq r ?

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