问题 选择题
椭圆C:
x2
a2
+
y2
b2
=1(a>b>0)
的左右焦点分别为F1,F2,若椭圆C上恰好有6个不同的点P,使得△F1F2P为等腰三角形,则椭圆C的离心率的取值范围是(  )
A.(
1
3
2
3
)
B.(
1
2
,1)
C.(
2
3
,1)
D.(
1
3
1
2
)∪(
1
2
,1)
答案

①当点P与短轴的顶点重合时,

△F1F2P构成以F1F2为底边的等腰三角形,

此种情况有2个满足条件的等腰△F1F2P;

②当△F1F2P构成以F1F2为一腰的等腰三角形时,

以F2P作为等腰三角形的底边为例,

∵F1F2=F1P,

∴点P在以F1为圆心,半径为焦距2c的圆上

因此,当以F1为圆心,半径为2c的圆与椭圆C有2交点时,

存在2个满足条件的等腰△F1F2P,

此时a-c<2c,解得a<3c,所以离心率e

1
3

当e=

1
2
时,△F1F2P是等边三角形,与①中的三角形重复,故e≠
1
2

同理,当F1P为等腰三角形的底边时,在e

1
3
且e≠
1
2
时也存在2个满足条件的等腰△F1F2P

这样,总共有6个不同的点P使得△F1F2P为等腰三角形

综上所述,离心率的取值范围是:e∈(

1
3
1
2
)∪(
1
2
,1)

阅读理解

It seems school children all over the world complain about their school food.Cherie Blair,the wife of Prime Minister Tony Blair,said that she would prepare a packed lunch for her son if school dinners do not improve. So what do students of your age eat for lunch at school?

Japan

High schools have canteens,which serve everything from noodles to rice,but not burgers and chips.Other children bring food from home such as cold rice balls,meat or fish,pickles and vegetables.

The United States

A typical menu from a US school is made up of a hamburger with fried potatoes or roast chicken,lettuce and pickles,fruit and cookies.School lunches must also provide at least one-third of the daily dietary

allowances(定量)of protein(蛋白质),vitamin A.vitamin C,iron,calcium,and calories(卡路里).

Australia

Meat pies,sausage rolls and hotdogs are all traditional dishes in Australian school shops.But as the nation pays more attention to children’s health,healthier foods have started to find their way onto school menus.

Many schools have used a traffic light system.The sale of red-labelled foods including pastries,chocolate and soft drinks,is served only twice a week.Healthier green-labelled foods such as sushi,sandwiches,corn and watermelon,however are served every day.

In some schools.students have a choice of up to 89 foods to choose from,including popcorn and rice.

South Africa

Most of South Africa’s schools do not serve meals at all.Classes end at 1:30 pm and students get their own lunches.Many students bring food from home,usually sandwiches.

Fast food and fried food sell the best among students,which has led to a rise in obesity among children.But as more people began to realize the fact that being too fat may cause different diseases,some schools in towns have led the way towards better nutrition(营养).Now students at these schools are provided with lunches of porridge with vegetables,such as cabbages,onions,beans,carrots and tomatoes.

小题1: Which belongs to red-labelled food in Australia?

A.sandwiches

B.beer

C.tomatoes

D.corn小题2:What does the underlined word “obesity” in the last paragraph probably mean?

A.Allowances.

B.Sadness.

C.Nutrition.

D.Overweight.小题3:We can infer(推断)from the passage that

A.a typical menu from a US school is made up of enough nutrition

B.most students in South Africa eat their lunches at home

C.many schools in Australia have traffic lights outside their schools

D.you can have whatever you like in school canteens in Japan小题4:What is the main idea of the passage?

A.Food served in the US is the best of a11.

B.Schools should try to satisfy the needs of students.

C.Schools serve different foods from country to country.

D.School children all over the world dislike their school food.小题5:.Who is the article meant for?

A.School lunch suppliers

B.Headmasters.

C.Students.

D.Nutritionists

选择题