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问题 选择题
若a，b∈R+，下列不等式中正确的是（　　）
A．(
a+b
2
)2≥ab≥
a2+b2
2
B．(
a+b
2
)2≥
a2+b2
2
≥ab
C．
a2+b2
2
≥(
a+b
2
)2≥ab		D．
a2+b2
2
≥ab≥(
a+b
2
)2
答案

令a=1，b=4

则 ab=4，(

a+b
2
)2= 
25
4
，
a2+b2
2
=
17
2
，

∴

a2+b2
2
≥ab≥(
a+b
2
)2

故选D．

单项选择题 A型题

在微生物的数码分类鉴定系统中，下列哪项试验通常于反应板（条）外进行（）

A.葡萄糖发酵试验

B.枸橼酸盐利用实验

C.β-半乳糖苷酶实验

D.氧化酶试验

E.H2S产生实验
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单项选择题

In a public-key cryptosystem, (71) encrypted with the public key can only be deciphered with the private key from the same (72) . Conversely, (71) encrypted with the private key can be decrypted only with the public key. Users can make their public keys (73) available or place them at a key distribution center or others to access. However, the private key must be (74) safe. In public-key systems there’s no (75) to find a safe channel for communicating a shared secret key.

A. more B. necessary C. need D. space
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