8. (2023春•涪城区校级月考)已知函数f(x)=x3+ax2−bx(a>0,b>0)f(x)=x^{3}+ax^{2}-bx(a>0,b>0)f(x)=x3+ax2−bx(a>0,b>0)在x=−1x=-1x=−1处有极值,则1a+2b\frac{1}{a}+\frac{2}{b}a1+b2的最小值为((( )))