1.4+2.5+3.6+...+99.102
=1(2+2)+2(3+2)+3(4+2)+...99(100+2)
=1.2+1.2+2.3+2.2+...+99.100+99.2
=(1.2+2.3+...+99.100)+2(1+2+...+99)
A=1.2+2.3+3.4+...+99.100(cho A la ten bieu thuc nay)
3A=1.2(3-0)+2.3(4-1)+...+99.100(101-98)
=(1.2.3+2.3.4+...+99.100.101)-(1.2.3+2.3.4+3.4.5+...+98.99.100
=99.100.101=>A=99.100.101399.100.1013=33330
2.(1+2+...99)
=2(100.99:2)=2.4950=9900
33330+9900=343200
Vậy B = 343200