Tính các tổng sau

1 ) -1 + 2 + ( -3 ) + 4 + ( -5 ) + . . . + 98 + ( -99 )
2 ) 1 + 3 + ( -5 ) + ( -7 ) + 9 + 11 + . . . + 97 + 99
3 ) ( -2 ) + ( -4 ) + 6 + 8 + ( -10 ) +( -12 ) + . . . + ( -98 ) + ( -100 )
4 ) 1 + ( -2 ) + ( -3 ) + 4 + 5 + ( -6 ) + ( -7 ) + 8 + 9 + . . . + ( -2015 ) + ( -2016 )