A=1.2+2.3+3.4+.....+100.101
3A=1.2.3+2.3.3+3.4.3+..+100.101.3
3A=1.2.3+2.3.(4-1)+3.4.(5-2)......+100.101.(102-99)
3A=1.2.3+2.3.4-2.3.1+3.4.5-3.4.2+....+99.100.101-100.101.102
3A=100.101.102
3A=\(\frac{100.101.101}{3}\)
Ta có : A = 1.2 + 2.3 + 3.4 + ..... + 100.101
=> 3A = 1.2.3 - 1.2.3 + 2.3.4 - 2.3.4 + ...... + 100.101.102
=> 3A = 100.101.102
=> A = 100.101.102/3
=> A = 343400