Mark M. answered 12/16/15
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Retired math prof. Very extensive Precalculus tutoring experience.
n(n+1)(n+2) = (n+3)(n)2 + 2n
Proof: (n+3)n2 + 2n = n[n(n + 3) + 2]
= n(n2 + 3n + 2)
= n(n + 1)(n + 2)