原式=x³-x-2x+2
=x(x²-1)-2(x-1)
=x(x+1)(x-1)-2(x-1)
=(x²+x)(x-1)-2(x-1)
=(x-1)(x²+x-2)
=(x-1)(x+2)(x-1)
=(x-1)²(x+2)