搜题请搜索小程序“爱下问搜题”
问题 选择题
对于使f（x）≤M恒成立的所有常数M中，我们把M的最小值叫做f（x）的上确界．若a＞0，b＞0且a+b=1，则-
1
2a
-
2
b
的上确界为（　　）
A．
9
2
B．-
9
2
C．
1
4
D．-4
答案

∵

1
2a
+
2
b
=
a+b
2a
+
2(a+b)
b
=
5
2
+
b
2a
+
2a
b
，（当且仅当 a=b=
1
2
时取到等号）

∴-

1
2a
-
2
b
≤-
9
2
（当且仅当 a=b=
1
2
时取到上确界）

故选B．

多项选择题

当预测期货价格将下跌，下列期权交易中正确的是( )。

A．买入看涨期权

B．卖出看涨期权

C．买入看跌期权

D．卖出看跌期权
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补全对话，情景问答
根据对话内容及方框中所给的句子补全对话, 选项中有两项是多余选项。
A. That sounds terrible!
B. Have you ever read it?
C. It is really a good novel.
D. I always forget.
E. I'm not sure yet.
F. No, I didn't.
G. That's very kind of you.
A: Where did you go?
B: I went to the library to borrow the book Gone with the Wind. 1. _____
A: No, I haven't.
B: That's a pity. 2. _____
A: Well, I've been thinking about borrowing a book from the library for a while.
3. _____
B: Well, look! Here it is. If you like, I can lend it to you after I finish reading.
A: 4. _____ Do you know when that will be?
B: 5. _____ I'll lend it to you as soon as I finish it.
A: By the way, how long can you keep it?
B: Two weeks.
A: That sounds good.
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