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)
