N = 1.2.3 + 2.3.4 + 3.4.5 + ... + 18.19.20
3.N = 1.2.3.3 + 2.3.4.3 + 3.4.5.3 + ... + 18.19.20.3
3.N = 1.2.3.(4-1) + 2.3.4.(5-2) + 3.4.5.(6-3) + ... + 18.19.20.(21-18)
3.N = 1.2.3 - 1.2.3 + 2.3.4 - 2.3.4 + 3.4.5 - 3.4.5 + .... - 17.18.19 + 18.19.20
3.N = 18.19.20
N = 18.19.20 : 3
N = 2280
Chúc học tốt!!!