Question

1x2+2x3+3x4+...+98x99+99x100 = ?

Answer 1

Đặt S = 1x2+2x3+3x4+...+98x99+99x100

S x 3 =1x2x3+2x3x3+3x4x3+...+98x99x3+99x100x3

S x 3 =1x2x(3-0)+2x3x(4-1)+3x4x(5-2)+....+98x99x(100-97)+99x100x(101-98)

S x 3 = 1x2x3 + 2x3x4-1x2x3+3x4x5-2x3x4+...+98x99x100-97x98x99+99x100x101-98x99x100

S x 3 = 99x100x101

S x 3 = 999900

S = 333300