n*(n+1)*(n+2)=n^3+3n^2+2n和=(1^3+2^3+....+n^3)+3*(1^2+2^2+....+n^2)+2*(1+2+...+n)=[n(n+1)/2]^2+3*n(n+1)(2n+1)/6+2*n(n+1)/2=n(n+1)(n+2)(n+3)/4
