Question

1.2.3+2.3.4+3.4.5+...+17.18.19+18.19.20

Answer 1

Đặt A = 1 . 2 . 3 + 2 . 3 . 4 + 3 . 4 . 5 + ... + 17 . 18 . 19 + 18 . 19  . 20

=> 4A = 1 . 2 . 3 . 4 + 2 . 3 . 4 . 4 + 3 . 4 . 5 . 4 + ... + 17 . 18 . 19 . 4 + 18 . 19 . 20 . 4

     4A = 1 . 2 . 3 . 4 + 2 . 3 . 4 . ( 5 - 1 ) + 3 . 4 . 5 . ( 6 - 2 ) + ... + 17 . 18 . 19 . ( 20 - 16 ) + 18 . 19 . 20 . ( 21 - 17 )

     4A = 1 . 2 . 3 . 4 + 2 . 3 . 4 . 5 - 1 . 2 . 3 . 4 + 3 . 4 . 5 . 6 - 2 . 3 . 4 . 5+ ... + 17.18.19.20 - 16.17.18.19 + 18.19.20.21 -17.18.19.20

     4A = 18 . 19 . 20 . 21

 => A = 18 . 19 . 20 . 21 : 4

      A = 35 910

Answer 2

Đặt M = 1.2.3+2.3.4 + 3.4.5+...+17.18.19+18.19.20

=> 4M = 1.2.3.4+2.3.4.4+3.4.5.4+...+17.18.19.4+18.19.20

4M = 1.2.3.(4-0)+2.3.4.(5-1)+3.4.5.(6-2)+...+17.18.19.(20-16)+18.19.20.(21-17)

4M = 1.2.3.4 + 2.3.4.5 - 1.2.3.4 + 3.4.5.6 - 2.3.4.5 + ...+17.18.19.20 - 16.17.18.19 + 18.19.20.21 - 17.18.19.20

4M = ( 1.2.3.4 + 2.3.4.5 + 3.4.5.6 + ...+ 17.18.19.20+18.19.20.21) - (1.2.3.4+2.3.4.5+...+16.17.18.19+17.18.19.20)

4M = 18.19.20.21

\(M=\frac{18.19.20.21}{4}\)