1-2-3+4+5-6-7+8+...+21-22-23+24
= ( 1-2-3 + 4) + ( 5-6-7+8)+...+(21 -22- 23 +24)
= 0+0 +....+0
= 0
1 - 2 - 3 + 4 + 5 - 6 - 7 + 8 + . . . + 21 - 22 - 23 + 24
= ( 1 - 2 - 3 + 4 ) + ( 5 - 6 - 7 + 8 ) + . . . + ( 21 - 22 - 23 + 24 )
= 0 + 0 + . . . + 0
= 0