0/0,洛必达两次,分子分母分别求导
1:[n*m*(1+mx)^(n-1)-m*n*(1+nx)^(m-1)]/2x
2:[n*m^2*(n-1)(1+mx)^(n-2)-m*(m-1)*n^2*(1+nx)^(m-2)]/2
=m^2*n(n-1)-n^2*m(m-1)=mn(n-m)