3n*(n+1) = n(n+1)(n+2) - (n-1)n(n+1)所以3S = (n+1)(n+2)(n+3) - 0S = (n+1)(n+2)(n+3) /3
公式：Sn=1*2+2*3+3*4+...+n(n+1)=n(n+1)(n+2)/3
