8-5 双曲线
1.(文)(2011•烟台调研)与椭圆x24+y2=1共焦点且过点P(2,1)的双曲线方程是( )
A.x24-y2=1 B.x22-y2=1
C.x23-y23=1 D.x2-y22=1
[答案] B
[解析] 椭圆的焦点F1(-3,0),F2(3,0),
由双曲线定义知2a=|PF1|-|PF2|
=2+32+1-2-32+1
=8+43-8-43=22,
∴a=2,∴b2=c2-a2=1,
∴双曲线方程为x22-y2=1.
(理)(2011•山东理,8)已知双曲线x2a2-y2b2=1(a>0,b>0)的两条渐近线均和圆C:x2+y2-6x+5=0相切,且双曲线的右焦点为圆C的圆心,则该双曲线的方程为( )
A.x25-y24=1 B.x24-y25=1
C.x23-y26=1 D.x26-y23=1
[答案] A
[解析] 依题意:⊙C方程为(x-3)2+y2=4,∴圆心C(3,0),半径r=2,∴双曲线的右焦点F2为(3,0),即c=3.又双曲线的渐近线方程为y=±bax,即bx±ay=0,
∴|3b|a2+b2=2,即b=2,∴a2=9-4=5,故选A.