Question

1 x 2 + 2 x 3 + 3 x 4 + ..... + 38 x 39 + 39 x 40 = ?

Giúp mình nha.

Answer 1

Đặt A = 1.2 + 2.3 + 3.4 + ... + 38.39 + 39.40

=> 3A = 1.2.3 + 2.3.(4 - 1) + 3.4.(5 - 2) + ... + 38.39.(40 - 37) + 39.40.(41 - 38)

           = 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ... + 38.39.40 - 37.38.39 + 39.40.41 - 38.39.40

           = 39.40.41 = 63960

=> A = 63960 : 3 = 21320

Answer 2

1 x 40 x (2 x 2) + (3x2) + (4x2) +...+ ( 39 x 2)

1 x 40 x (2 + 3 + 4 + .... + 39) x 2

40    x 779 x 2

31160 x 2

62320

k nha