Question

1.2 + 2.3 + 3.4 + 4.5 + ... + 99.100

Answer 1

Gọi A = 1.2 + 2.3 + 3.4 +...+ 99.100

  3A = 1.2.(3 - 0) + 2.3.(4-1) + 3.4.(5-2) +...+ 99.100.(101-98)

    = 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 +...+ 99.100.101 - 89.99.100

   = 99.100.101 = 999900

A = 999900 : 3 = 333300

vậy A = 333300