分组分解法.
x³－6x²＋11x－6
=(x³－6x²＋5x)＋(6x－6)
=x(x²－6x＋5)＋6(x－1)
=x(x－5)(x－1)＋6(x－1)
=(x－1)(x²－5x＋6)
=(x－1)(x－2)(x－3)