问题 选择题
在△ABC中,角A、B、C所对的边为a,b,c,若a2、b2、c2成等差数列,则角B的最值及取最值时三角形面积为(  )
A.角B有最小值,此时s=
1
2
a2
B.角B有最大值,此时s=
1
2
ac
C.角B有最小值,此时s=
3
4
b2
D.角B有最大值,此时s=
3
4
a2
答案

由题意可得 2b2=a2 +c2 ,由余弦定理可得 cosB=

a2+c2-b2
2ac
=
a2+c2
4ac
1
2
,当且仅当a=c时,等号成立.

又 0<B<π,∴0<B≤

π
3
,即角B的最大值为
π
3

此时面积S=

1
2
acsin
π
3
=
3
4
a2

故选D.

多项选择题
单项选择题

There (1) not one type of reading but several according to your reasons for reading. To read carefully, you have to (2) your reading speed and technique (3) your aim (4) reading. Skimming is a technique necessary for quick and efficient reading.

When skimming, you (5) the reading (6) quickly in order to get the (7) of it, to know how it is organized, (8) an idea of the tone or the intention of the writer. Skimming is (9) an activity which (10) an overall view of the text and (11) a definite reading competence.

Skimming doesn’t need reading all the material, but it doesn’t mean that it is an (12) skill for the lazy, because it need a high degree of alertness and concentration.

When you read, you usually start with (13) understanding and move towards detailed understanding rather than working the other way round. But (14) is also used after you have already carefully studied and you need to (15) the major ideas and concepts.

In order to be able to skim quickly and (16) through a text, you should know where to look for what you want. In preview skimming you read the introductory information, the headings and subheadings, and the summary, if one is provided. (17) this skimming, decide whether to read the material more thoroughly, and select the appropriate speed (18) you read.

The same procedure (19) for preview skimming could also be used to get an overview. Another method would be to read only key words. This is done by omitting the unnecessary words, phrases, and sentences.

In order to skim efficiently and fulfill your purpose, (20) practice is necessary.

20().

A.a number of

B.an amount of

C.many

D.a few