若a,b为正实数,满足ab=a+b+3,求ab的范围.∵a>0,b>0,∴ab=a+b+3>3.令ab=u,则b=u/a,代入ab=a+b+3,得：u=a+u/a+3=(a²+3a+u)/a故a²+(3-u)a+u=0由于a为实数,故其判别式：△=（3-u）²-4u=u²-10u+9=(u-...