设直线y=-x+c,与y=±b/ax联列,
得Xa=c/(b/a+1),Xb=c/(-b/a+1),
又A为BF2中点,则XF2-Xb=2(XF2-Xa),
化简得3abc=b²c,即b=3a
则双曲线的渐近线方程为y=±b/ax=±3x