(x-2)(x-3)分之1减(x-1)(x-3)加(x-1)(x-2)分之1
=(x-1)(x-2)(x-3)分之【x-1-x+2+x-3】
=(x-1)(x-2)(x-3)分之（x-2）
=(x-1)(x-3)分之1不好意思少打了几个字，应该(x-1)(x-3)分之2(x-2)(x-3)分之1减(x-1)(x-3)分之2加(x-1)(x-2)分之1
=(x-1)(x-2)(x-3)分之【x-1-2x+4+x-3】
=(x-1)(x-2)(x-3)分之0
=0