`A= 1.2+2.3+3.4+...+n.(n+1)`
=> `3A = 1.2.3+ 2.3.3 + 3.4.3+....+ n(n+1).3`
=> `3A= 1.2.3 + 2.3.(4-1) + 3.4.(5-2) +.....+ n.(n+1). [(n+2) - ( n-1)]`
=> `3A= 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 -2.3.4 +....+ n(n+1)(n+2) -(n-1).n(n+1)`
=>` 3A = n.(n+1)(n+2) `
=> `A= [n.(n+1)(n+2) ]:3`
Vậy......