约分：①5ab/20a2b= _ ，②x2-9x2-6x+9= _ ．
问题描述：
约分：①
= ___ ，②5ab 20a2b
= ___ ．
x2-9
x2-6x+9
答
①
=5ab 20a2b
；1 4a
②
=
x2-9
x2-6x+9
=(x+3)(x-3) (x-3)2
．x+3 x-3