化简-x平方+9x-20分之x平方-7x+12
问题描述：
化简-x平方+9x-20分之x平方-7x+12
答
解
(x²-7x+12)/(-x²+9x-20)
=(x²-7x+12)/[-(x²-9x+20)]
=-(x²-7x+12)/(x²-9x+20)
=-(x-3)(x-4)/(x-5)(x-4)
=-(x-3)/(x-5)