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1.2.3+2.3.4+.....+n(n+1)(n+2) = ∑n(n+1)(n+2)
Applying ∑ to every number
=∑n³+ ∑3n²+∑2n
=n²(n+1)²/4 + 3{n(n+1)(2n+1)/6 +2n(n+1)/2
=n(n+1)/2{n(n+1)/2+ (2n+1)+2}
=n(n+1)/2{(n²+n)+ (4n+2)+4}/2
2 5 2
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