Simplifying a summation

The original problem states $\sum_{i=1}^k n_i = n$. However, I'm having trouble trying to figure out how to simplify this: $\sum_{i=1}^k n_i^2$.

I've been trying it and I don't think I can just simply write $n^2$, so can anyone point me in the right direction?

Thanks!

• There is no relation bewteen the sum of numbers and the sum of their squares, except for the trivial case $k=1$.
– user65203
Sep 5 '16 at 15:08

Without knowing something about the $n_i$ you can't do anything. Compare $k=2$ with $(n_1,n_2)= (3,3)$ and $(1,5)$