问题
选择题
过点P(-3,0)的直线l与双曲线
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答案
∵点P(-3,0)的直线l与双曲线
-x2 16
=1交于点A,B,直线l的斜率为k1(k1≠0),y2 9
设A(x1,y1),B(x2,y2),则k1=
,y2-y1 x2-x1
,①-②得:
-x12 16
=1①y12 9
-x22 16
=1②y22 9
-x12-x22 16
=0,即y12-y22 9
-(x1+x2)(x1-x2) 16
=0③;(y1+y2)(y1-y2) 9
设AB的中点M(x0,y0),则x1+x2=2x0,y1+y2=2y0,
又k1=
,代入③得:y2-y1 x2-x1
=k1•9 16
,又k2=y0 x0
=y0-0 x0-0
,y0 x0
∴k1•k2=
.9 16
故选A.