Question

1.2.3+2.3.4+...+98.99.100

Answer 1

Đặt S=1.2.3+2.3.4+...+98.99.100

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

4S=1.2.3.4-0.1.2.3+2.3.4.5-1.2.3.4+...+98.99.100.101-97.98.99.100

4S=(1.2.3.4+2.3.4.5+...+98.99.100.101) - (0.1.2.3+1.2.3.4+...+97.98.99.100)

4S=98.99.100.101-0.1.2.3

4S=98.99.100.101

S=98.99.25.101

S=24497550