问题 选择题
过点P(-3,0)的直线l与双曲线
x2
16
-
y2
9
=1
交于点A,B,设直线l的斜率为k1(k1≠0),弦AB的中点为M,OM的斜率为k2(O为坐标原点),则k1•k2=(  )
A.
9
16
B.
3
4
C.
16
9
D.16
答案

∵点P(-3,0)的直线l与双曲线

x2
16
-
y2
9
=1交于点A,B,直线l的斜率为k1(k1≠0),

设A(x1,y1),B(x2,y2),则k1=

y2-y1
x2-x1

x12
16
-
y12
9
=1①
x22
16
-
y22
9
=1②
,①-②得:
x12-x22
16
-
y12-y22
9
=0
,即
(x1+x2)(x1-x2)  
16
-
(y1+y2)(y1-y2)  
9
=0
③;

设AB的中点M(x0,y0),则x1+x2=2x0,y1+y2=2y0

又k1=

y2-y1
x2-x1
,代入③得:

9
16
=k1
y0
x0
,又k2=
y0-0
x0-0
=
y0
x0

∴k1•k2=

9
16

故选A.

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