Solve The Series

$$S_n = \sum_{k=1}^ {4n} {(-1)^\frac{k(k+1)}{2}} {k^2}$$.

Then Sn can take what values among:
1056, 1088, 1120, 1332

tiny-tim
Homework Helper
hi klen!

simplest way is probably to do the sum for 4m-3,4m-2,4m-1,4m (only) first …

what do you get?

HallsofIvy
$$S_n = \sum_{k=1}^ {4n} {(-1)^\frac{k(k+1)}{2}} {k^2}$$.