(n+1)(n+2)/1 +(n+2)(n+3)/1 +(n+3)(n+4)/1
=(n+1)(n+2) +(n+2)(n+3) +(n+3)(n+4)
=(n+2)(n+1+n+3)+n^2+7n+12
=(n+2)(2n+4)+n^2+7n+12
=2(n+2)^2+n^2+7n+12
=2(n^2+4n+4)+n^2+7n+12
=3n^2+15n+20
(n+1)(n+2)/1 +(n+2)(n+3)/1 +(n+3)(n+4)/1
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