设弦为AB.斜率为k,A(x1,y1)B(x2,y2),代入双曲线,两式相减得(x1+x2)/9=(y1+y2)(y1-y2)/4(x1-x2)即4/9=2k/4,得k=8/9所以方程为y-1=8/9(x-2)即8x-9y-7=0