Question

Tính A=1x2+2x3+3x4+....+999x1000

Answer 1

A = 1.2 + 2.3 + .... + 999.1000

3A = 1.2.3 + 2.3.(4-1) + .... + 999.1000.(1001 - 998)

3A = 1.2.3 + 2.3.4 - 1.2,3 +..... + 999.1000.1001 - 998.999.1000

3A = 999 . 1000 . 1001

A = 333 x 1000 x 1001 = 333 333 000

Answer 2

3A=1x2x3+2x3x(4-1)+3x4x(5-2)+.............+999x1000x(1001-998)

3A=1x2x3+2x3x4-1x2x3+3x4x5-2x3x4+............+999x1000x1001-998x999x1000

3A=999x1000x1001

A=999x1000x1001:3

A=333333000

Answer 3

=> 3A = 1.2.3 + 2.3.(4 - 1) + 3.4.(5 - 2) + ................ + 999.1000.(1001 - 998)

=> 3A = 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ............... + 999.1000.1001 - 998.999.1000

=> 3A = 999.1000.1001

=> A = 999.1000.1001 = 333333000