Question

 1.2.3+2.3.4+.........+98.99.100 = ?

Answer 1

Đặt A=1.2.3+2.3.4+...+98.99.100

=>4.A=1.2.3.4+2.3.4.4+...+98.99.100.4

=>4.A=1.2.3.(4-0)+2.3.4.(5-1)+...+98.99.100.(101-97)

=>4.A=1.2.3.4-0.1.2.3+2.3.4.5-1.2.3.4+...+98.99.100.101-97.98.99.100

=>4.A=98.99.100.101

=>4.A=97990200

=>A=24497550

Vậy 1.2.3+2.3.4+...+98.99.100=24497550