(x^2-4x-12)(x^2-4x+3)+56
问题描述：
(x^2-4x-12)(x^2-4x+3)+56
答
令a=x^2-4x
原式=(a-12)(a+3)+56
=a²-9a-36+56
=a²-9a+20
=(a-4)(a-5)
=(x^2-4x-4)(x^2-4x-5)
=(x^2-4x-4)(x-5)(x+1)