1x2+2x3+3x4+...+99x100
=(99.100.101-0.1.2):3
=333300
Đặt A=1.2+2.3+3.4+............+99.100
3A=1.2.3+2.3.(4-1)+3.4.(5-2)+.............+99.100.(101-98)
3A=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+................+99.100.101-98.99.100
3A=99.100.101
A=99.100.101:3
A=333300
1 x 2 + 2 x 3 - 3 x4 +...+ 99 x 100
= ( 9.100.101-0.1.2 ) : 3
=333300