分子:(m+1)(m+3)(2m-m^2)=m(m+1)(m+3)(2-m)=-m(m+1)(m+3)(m-2)
分母:(m^3+m^2)(m-2)(m+3)=m^2(m+1)(m-2)(m+3)
进行约分得:(-m)/m^2,将m的指代入得4/3