(x^2+mx+n)(2x-1)=2x^3+2mx^2+2nx-x^2-mx-n
=2x^3+(2m-1)x^2+(2n-m)x-n
若干上式不含有x的平方项和x项,
则2m-1=0,2n-m=0
m=1/2,n=1/4