Question

Tính: S = 1 x 2 + 2 x 3 + 3 x 4 + 38 x 39 + 39 x 40 

Answer 1

S = 21320

Answer 2

Ta có:
S = 1x2 + 2x3 + 3x4 + ...+ 38x39 + 39x40
S x 3 = 1x2x3 + 2x3x3 + 3x4x3 +… + 38x39x3 + 39x40x3
S x 3 = 1x2x3 + 2x3x(4-1) + 3x4x(5-2) + ... + 38x39x(40-37) + 39x40x(41-38)
S x 3 = 1x2x3 + 2x3x4 - 1x2x3 + 3x4x5 - 2x3x4 + ... + 38x39x40 - 37x38x39 + 39x40x41 - 38x39x40.
S x 3 = 39x40x41
S = 39x40x41:3= 21320

Answer 3

ta có: S=2+6+12+1482+1560

 S=3062

chúc bạn học giỏi,nhớ k cho mình

Answer 4

 bằng 3062