Question

TINH

A=1.2+2.3+ 3.4+...+99.100

Answer 1

3A = 1.2.(3-0) + 2.3.(4-1) + 3.4.(5-2) + ... + 99.100.(101-98)

3A = 1.2.3 - 0.1.2 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ... + 99.100.101 - 98.99.100

3A = 99.100.101 - 0.1.2

3A = 99.100.101

A = 33.100.101

A = 333300