S=1.2+2.3+3.4+4.5+....+99.100
3S=1.2.3+2.3.3+3.4.3+4.5.3+....+99.100.3
3S=1.2.3+2.3.(4-1)+3.4.(5-1)+4.5(6-3)+....+99.100(101-98)
3S=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+4.5.6-2.4.5+....+99.100.101-98.99.100
3S=1.2.3-1.2.3+2.3.4-2.3.4+3.4.5-3.4.5+4.5.6-4.5.6+.......+99.100.101
3S=99.100.101
3S=999900
S=999900:3
S=333300
S=1.2+2.3+3.4+4.5+...+99.100
3S=1.2.3+2.3.(4-1)+3.4.(5-2)+4.5.(6-3)+...+99.100.(101-98)
3S=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+4.5.6-3.4.5+...+99.100.101-98.99.100
3S=99.100.101
S=(99.100.101):3=333300
S=1.2+2.3+3.4+4.5+...+99.100
=>3S=1.2.3+2.3.3+3.4.3+4.5.3+...+99.100.3
=1.2.3+2.3.﴾4‐1﴿+3.4.﴾5‐2﴿+4.5.﴾6‐3﴿+...+99.100.﴾101‐98﴿
=1.2.3+2.3.4‐1.2.3+3.4.5‐2.3.4+4.5.6‐3.4.5+...+99.100.101‐98.99.100
=1.2.3‐1.2.3+2.3.4‐2.3.4+3.4.5‐3.4.5+4.5.6‐4.5.6+...+99.100.101
=99.100.101=999900
=>S=999900:3=333300
Vậy S=333300