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An= Sn+T(n-1)
((n²+n)/2)*n = Sn + Sn/2 -n²/4 -n/4
(3/2)Sn = ((n²+n)/2)*n +n²/4 +n/4
(3/2)Sn =(1/2)(n³+n²+n²/2+n/2)
Sn = n³/3 +n²/2 +n/6
A= base*altezza
= ((n²+n)/2)*n