y=（2x^2-x+1)/(2x-1)
=[x(2x-1)+1]/(2x-1)
=x+1/(2x-1)
=x-1/2+[(1/2)/(x-1/2)]+1/2
因为x>1/2
所以y≥2根号{(x-1/2)×[(1/2)/(x-1/2)]}+1/2
即≥1/2+根号2