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