（a^2-7a+6)(a^2-a-6)+56
问题描述：
（a^2-7a+6)(a^2-a-6)+56
一定要用换元啊
答
=(a-1)(a-6)(a-3)(a+2)+56
=[(a-1)(a-3)][(a-6)(a+2)]+56
=[(a²-4a)+3][(a²-4a)-12]+56
令x=a²-4a
=(x+3)(x-12)+56
=x²-9x+20
=(x-4)(x-5)
=(a²-4a-4)(a²-4a-5)
=(a²-4a-4)(a-5)(a+1)