（1）原式
=(x³-1)-(11x³-31x+20)
=(x-1)(x²+x+1)-(11x-20)(x-1)
=(x-1)(x²+x+1-11x+20)
=(x-1)(x²-10x+21)
=(x-1)(x-3)(x-7)
（2）原式
=x³-2x²+4
这个没法分解吧,我想不出怎么分解了
（3）原式
=x³-2x²-x²+4
=x²(x-2)-(x²-4)
=x²(x-2)-(x+2)(x-2)
=(x-2)[x²-(x+2)]
=(x-2)(x²-x-2)
=(x-2)(x-2)(x+1)
=(x+1)(x-2)²