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问题 选择题
已知正数x、y满足x+y=1，则
1
x
+
4
y
的最小值是（　　）
A．7B．8C．9D．10
答案

∵正数x、y满足x+y=1，

∴

1
x
+
4
y
=（x+y）（
1
x
+
4
y
）

=1+

y
x
+
4x
y
+4

≥5+2

y
x
×
4x
y

=9．

当且仅当

y
x
=
4x
y
，即x=
1
3
，y=
2
3
时，
1
x
+
4
y
的最小值是9．

故选C．

改错题

短文改错（共10小题；每小题1分，满分10分）

An Australian farmer found the kangaroo caught          小题1:          

in the fence around his farms. He thought the poor         小题2:         

animal was injured, but the kind farmer took off his       小题3:          

jacket and put on the animal. Then he started trying      小题4:          

to cut the fence to free the animal. But as soon as        小题5:          

the kangaroo were free, it jumped up and ran away          小题6:          

with the jacket. The farmer was worried because of his     小题7:          

wallet was in the pocket. But, to my surprise, when        小题8:         

he gets home, he saw the animal waiting at the door,                   小题9:          

sill wore his jacket with the wallet in the pocket.      小题10:        
查看答案
单项选择题

免疫自稳功能是指（）

A.机体识别排除抗原性异物的功能

B.机体识别排除病原体的功能

C.机体识别排除衰老死亡细胞的功能

D.机体识别排除胞内寄生物的功能

E.机体识别排除突变细胞的功能
查看答案