1-2-3-4+...+1997-1998+1999-2000=

Question

A) -2000
B) -1000
C) -1
D) 0

Answer 1

I think you have a typo and meant

1 - 2 + 3 - 4 + ... + 1997 - 1998 + 1999 - 2000
(all odds are positive and all evens are negative)

take them in pairs:
1-2 = -1
3-4 = -1
5-6 = -1
..
1999-2000=-1
so you have 1000 of these pairs, and the sum is -1000