Find the sum of n terms of the series
`1*2*3+2*3*4+3*4*5...........`

Explanation

The given series is `1 xx 2 xx 3+2 xx 3 xx 4+3 xx 4 xx 5+.....`


`n^(th)`term,`a_n=n(n+1)(n+2)`


`=(n^2+n)(n+2)`


`=n^3+3n^2+2n`


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