化简分式（必须是最简形式
问题描述：
化简分式（必须是最简形式
(1)m^2-3m/9-m^2
(2)x^2-7x/49-x^2
答
(1)m^2-3m/9-m^2
=-(3-m)*m/(3-m)*(3+m)
=-m/(3-m)
(2)x^2-7x/49-x^2
=-x*(7-x)/(7+x)*(7-x)
=-x/(7+x)