Tính tổng:

Tính tổng:

1/ 1 + (-2) + 3 + (-4) + . . . + 19 + (-20)

2/ 1 – 2 + 3 – 4 + . . . + 99 – 100

3/ 2 – 4 + 6 – 8 + . . . + 48 – 50

4/ – 1 + 3 – 5 + 7 - . . . . + 97 – 99

5) 1 + 2 – 3 – 4 + . . . . + 97 + 98 – 99 - 100