`A = 1.2 + 2.3 + 3.4 + ... + n(n+1)`
`=> 3A = 1.2.3 + 2.3.3 + 3.4.3 + ... + n(n+1).3`
` = 1.2.3 + 2.3(4-1) + 3.4.(5-2) + ... + n(n+1)(n+2 - n-1 )`
`=> 3A - A = 2A = 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 1.2.3 + ... n(n+1)(n+2) - (n-1)n(n+1) `
` => 2A = (n-1)n(n+1)`
` => A = \frac{(n-1)n(n+1)}{2}`
