A= 1.2 +2.3+ 3.4 +4.5+...+ 99.100
3A = 1.2.3 +2.3.3+ 3.4.3+...+99.100.3
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 = 999900
A = 333300
A= 1. 2 + 2.3 +3.4 + 4.5 + 99.100
A=2 +6+12+20 + 9900
A = 9930
bn nha
tk mk nha
A=1.2+2.3+3.4+4.5+99.100
A=101x101x102:3
A=343400
k nha