由x²+3x-1=0,可得：x²+3x=1,
由x³+5x³+5x+8
=x³+3x²+2x²+5x+8
=x（x²+3x）+2x²+5x+8
=x+2x²+5x+8
=2x²+6x+8
=2（x²+3x）+8
=2+8
=10.