Tính tổng S = 1.2 + 2.3 + 3.4 +.... + 99.100.

S = 1 . 2 + 2 . 3 + ... + 99 . 100

3S = 1 . 2 . 3 + 2 . 3 . (4 – 1) + ..... + 99 . 100 . (101 – 98)

3S = 1 . 2 . 3 + 2 . 3 . 4 – 1 . 2 . 3 + .... + 99 . 100 . 101 – 98 . 99 . 100

3S = 99 . 100 . 101

3S = 999900

S = 999900 : 3

S = 333300.