Compute the sum (-1) + 3 + (-5) + 7 + ... + (99) + (-101).
Note that we can write
(-1 + 3) + (-5 + 7) + ( -9 + 11) + .....+ ( -97 + 99) + (-101)
(2) + (2) + (2) + .......+ (2) + (-101)
The number of summed 2's = [ 99 - (- 1) ] / 4 = 100/4 = 25
So...the sum is
25(2) - 101 =
50 - 101 =
-51