2,2+4,2+4+6,……,2+4+6+……+2n,……,求S(n)1+(1+2)+(1+2+3)+...+(1+2+3+...n) =1*n+2*(n-1)+3*(n-2)+...+(n-1)*[n-(n-2)]+n*[n-(n-1)] =(n+2n+3n+...n*n)-[2*(2-1)+3*(3-1)+4*(4-1)+...+n*(n-1)] =n*n*(n+1)/2-(2*...