Ta có: 10+20+30+40+...1980.123
= 10+20+30+40+....+243540
= 10+2x10+3x10+4x10+...+24354x10
= 10x(1+2+3+4+...+24354)
= 10x296570835
= 2965708350
ban oi sao lai 1980,123
10+2.10+1.10+4.10+.....+24354.10
10.(1+2+3+4+........+24354)
10.296570835=2965708350