x的2次方-3x-1=0
则x²-3x=1
2x的3次方-3x的2次方-11x+8
=2x(x²-3x)+3x²-11x+8
=2x×1+3x²-11x+8
=3x²-9x+8
=3(x²-3x)+8
=3×1+8
=11