问题 解答题
已知椭圆
x2
a2
+
y2
b2
=1(a>b>0)
上有一个顶点到两个焦点之间的距离分别为3+2
2
3-2
2

(1)求椭圆的方程;
(2)如果直线x=t(t∈R)与椭圆相交于A,B,若C(-3,0),D(3,0),证明直线CA与直线BD的交点K必在一条确定的双曲线上;
(3)过点Q(1,0)作直线l(与x轴不垂直)与椭圆交于M、N两点,与y轴交于点R,若
RM
MQ
RN
NQ
,证明:λ+μ为定值.
答案

(1)由已知得

a+c=3+2
2
a-c=3-2
2
,解得
a=3
c=2
2

∴b2=a2-c2=1…(3分)

∴椭圆方程为

x2
9
+y2=1.…(5分)

(2)依题意可设A(t,y0),B(t,-y0),K(x,y),且有

t2
9
+y02=1

CA:y=

y0
t+3
(x+3),DB:y=
-y0
t-3
(x-3),

y2=

-
y20
t2-9
(x2-9),

t2
9
+y02=1代入即得y2=
1
9
(x2-9),
x2
9
-y2=1

所以直线CA与直线BD的交点K必在双曲线

x2
9
-y2=1上.…(10分)

(3)依题意,直线l的斜率存在,故可设直线l的方程为y=k(x-1),…(11分)

设M(x3,y3)、N(x4,y4)、R(0,y5),则M、N两点坐标满足方程组

y=k(x-1) 
x2
9
+y2=1 .

消去y并整理,得(1+9k2)x2-18k2x+9k2-9=0,

所以x3+x4=

18k2
1+9k2
,①x3x4=
9k2-9
1+9k2
,②…(13分)

因为

RM
MQ
,所以(x3,y3)-(0,y5)=λ[(1,0)-(x3,y3)],

x3=λ(1-x3
y3-y5=-λy3 .
,所以x3=λ(1-x3),

又l与x轴不垂直,所以x3≠1,

所以λ=

x3
1-x3
,同理μ=
x4
1-x4
.        …(14分)

所以λ+μ=

x3
1-x3
+
x4
1-x4
=
(x3+x4)-2x3x4
1-(x3+x4)+x3x4

将①②代入上式可得λ+μ=-

9
4
.      …(16分)

单项选择题
完形填空
完形填空
I came to study in the US last fall. My life had been quite  1 since I arrived at
Andover. Although I had quite a few extracurricular (课外的) activities, I hadn’t taken any official leadership position.
This  2 changed when I received an e-mail from Ms Griffith, my cluster dean (主
任),    3 that a few female DC Rep was needed. DC Rep 4 Discipline Committee Representative.  5 boarding school students break serious rules, like using  6  , getting drunk or cheating in the studies. The punishments given to such students are   7 by a DC Rep, a house counselor, two student representatives, the cluster dean. I was really interested in becoming a DC Rep for my cluster, Pine Knoll, so were five other students. We had to hold   9 and Ms Griffith asked us to write  10  to be sent to everyone in Pine Knoll.
I thought hard about what to write. I didn’t  want to make any   11 because they are difficult to keep. I didn’t want to say I was experienced, because I was not. So I started my speech by explaining  12  I wanted to become a DC Rep. “ I want to serve Pine Knoll and also learn how American boarding schools carry out disciplines,” I wrote  13 .
I made three   14 that said “ Vote For Teresa” and put them on every exit (出口处) of my dorm so that people would  15 my name when they went to vote.
Although I tried hard, I didn’t win the election, I was  16 . But my positive nature didn’t allow me to   17 for very long. I actually  18 when my friends tried to comfort(安慰) me. “Teresa, I’m sorry that you didn’t win the election,” they would say.
“ No, don’t be sorry for me. I am very happy to have taken part in the election.” I said.
It is true: the   19 is much more important than the result. I am surprised and happy to discover my hidden  20 .
小题1:
A.hardB.terribleC.happyD.simple
小题2:
A.conditionB.situationC.timeD.life
小题3:
A.explainingB.writingC.sayingD.talking
小题4:
A.stands forB.asks forC.calls forD.looks for
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A.UsuallyB.SometimesC.oftenD.Sometimes
小题6:
A.computersB.drugsC.carsD.mobiles
小题7:
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小题8:
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小题9:
A.a discussionB.a meetingC.an electionD.a selection
小题10:
A.articlesB.speechesC.compositionsD.introductions
小题11:
A.mistakesB.movementsC.promisesD.requests
小题12:
A.howB.whetherC.whenD.why
小题13:
A.seriouslyB.easilyC.fluentlyD.honestly
小题14:
A.postersB.advertisementsC.speechesD.promises
小题15:
A.recognizeB.rememberC.forgetD.know
小题16:
A.satisfiedB.surprisedC.disappointedD.helpless
小题17:
A.put outB.look downC.feel downD.lie down
小题18:
A.smiledB.criedC.laughedD.sorrowed
小题19:
A.reasonB.attitudeC.processD.aim
小题20:
A.strengthB.courageC.abilityD.skill