Question

1x2+2x3+3x4+...+1999x2000

Answer 1

HOC QUA NHUNG QUEN MAT ROI 

Answer 2

Đặt A=1x2+2x3+3x4+.......+1999x2000

3xA=1x2x3+2x3x3+3x4x3+...........+1999x2000x3

3xA=1x2x3+2x3x(4-1)+............+1999x2000x(2001-1998)

3xA=1x2x3+2x3x4-1x2x3+...........+1999x2000x2001-1998x1999x2000

3xA=1999x2000x2001

A=1999x2000x2001:3

A=2666666000

Answer 3

2666666000 li ke cho mk nha