an=n^2
= n(n+1) -n
= (1/3)[ n(n+1)(n+2) - (n-1)n(n+1) ] - (1/2) [ n(n+1) -(n-1)n ]
Sn = a1+a2+...+an
=(1/3)n(n+1)(n+2) - (1/2)n(n+1)
= (1/6)n(n+1)(2n+1)