=(x-1)(x-4)(x-2)(x-3)
=(x²-5x+4)(x²-5x+6) 把x²-5x看成一个整体 相当于(x+4)(x+6)
=(x²-5x)²+10(x²-5x)+24
=x的4次方-10x³+35x²-50x+24