Question

Tinh A= 1+3+6+10+...+4851+4950=?

Answer 1

A=1+3+6+10+...+4851+4950
2A=2+6+12+20+...+9702+9900
2A=1.2+2.3+3.4+4.5+...+98.99+99.100
Xét B=1.2+2.3+3.4+4.5+...+98.99+99.100
3B=1.2.3+2.3(4−1)+3.4(5−2)+...+99.100(101−98)
3B=1.2.3+2.3.4−1.2.3+3.4.5−2.3.4+...+99.100.101−98.99.100
3B=99.100.101
B=333300
Thay B vào A ta được:
2A=333300
A=166650