Bạn tham khảo :
$1$
$ 1 + (-2) + 3 + (-4) + . . . + 19 + (-20)$$= [1 + (-2) ]+[ 3 + (-4)] + . . . + [19 + (-20)]$
$= (-1) + (-1) + ... + (-1)$
$= (-1) . 10$
$= (-10)$
$2$
$1 – 2 + 3 – 4 + . . . + 99 – 100$
$= (1-2)+ (3-4) + ... + (99-100)$
$= (-1) + (-1) +... + (-1)$
$= (-1) . 50$
$= (-50$
$3$
$2 – 4 + 6 – 8 + . . . + 48 – 50$
$=( 2 – 4) + (6 – 8) + . . . + (48 – 50)$
$= (-2) + (-2) + ... + (-2)$
$= (-2) .25$
$= (-50)$
$4$
$– 1 + 3 – 5 + 7 - . . . . + 97 – 99$
$ =( -1 - 99 ) + (3 + 97) + ... + (49+51)$
$= -100 + 100 + .... + 100$
$= 0 + ... + 100$
$= 100$
$5$
$1 + 2 – 3 – 4 + . . . . + 97 + 98 – 99 - 100$
$=(1 + 2 – 3 – 4) + . . . . + (97 + 98 – 99 - 100)$
$= (-4) + ... + (-4)$
$= (-4) .25$
$= -100$
