A-B是:
(x³-2x²+1)-(2x²-3x-1)
=x³-2x²+1-2x²+3x+1
=x³-4x²+2
2x²+()-8x²-1
=2x³+7x-3
=2x³+7x-3+1+8x²- 2x²
=2x³+6x²+7x-2