问题 解答题
已知曲线C1:ρcos(θ+
π
4
)=
2
2
;曲线C2:ρ2=
3
2-cos2θ

(1)试判断曲线C1与C2的交点个数;
(2)若过点M(1,0)直线l与曲线C2交于两个不同的点A,B,求
|MA|•|MB|
|AB|
的取值范围.
答案

(1)由ρcos(θ+

π
4
)=
2
2
,得
2
2
ρ(cosθ-sinθ)=
2
2

所以x-y=1,

由ρ2=

3
2-cos2θ
,得ρ2(3-2cos2θ)=3,

所以3(x2+y2)-2x2=3,即x2+3y2=3,

x-y=1
x2+3y2=3
得2x2-3x=0,解得x=0或x=
3
2

所以曲线C1与C2的交点有两个;

(2)①当直线l存在斜率时,设l的方程为y=k(x-1),A(x1,y1),B(x2,y2),

y=k(x-1)
x2+3y2=3
得(1+3k2)x2-6k2x+3k2-3=0,

△=36k4-4(1+3k2)(3k2-3)>0,即2k2+1>0恒成立,

x1+x2=

6k2
1+3k2
x1x2=
3k2-3
1+3k2

|MA|=

1+k2
|x1-1|,|MB|=
1+k2
|x2-1|
,|AB|=
1+k2
|x1-x2|

|MA|•|MB|
|AB|
=
(1+k2)|x1-1||x2-1|
1+k2
|x1-x2|
=
1+k2
|x1x2-(x1+x2)+1|
|x1-x2|

=

1+k2
|
3k2-3
1+3k2
-
6k2
1+3k2
+1|
(
6k2
1+3k2
)2-
4(3k2-3)
1+3k2
=
6
6
k2+1
k2+
1
2
=
6
6
1+
1
2k2+1

又k2≥0,所以

6
6
|MA|•|MB|
|AB|
6
6
2
=
3
3

②当直线l不存在斜率时,把x=1代入x2+3y2=3得y=±

6
3

此时

|MA|•|MB|
|AB|
=
(
6
3
)2
2
6
3
=
6
6

综合①②得

|MA|•|MB|
|AB|
的取值范围为[
6
6
3
3
].

单项选择题
判断题