Hi,
I need a little bit help with this:
Got to work my way from "sum from k=1 to n (k^2*n nCr k))" to this: (n^2+n)*2^(n-2), but HOW?
I´ve tried god-knows-what but nothing seems to work..
Did you try calculating it for a couple of different values of n? When n= 1, the sum is $\displaystyle 1^2(1)(1)(1)= 1$. The right side you want to show that equal to is $\displaystyle (1^2+ 2)(2^{-1})= \frac{3}{2}$.
You can't prove that, it isn't true.
(Perhaps I am confused as to what "nCr" means. I thought that was the binomial coefficient but there is no "r" on the right, and it isn't an index so I assumed you meant "nCk".)