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问题 填空题
如果向量
a
=(λ，2)，
b
=(-3，5)，且
a
，
b
的夹角是钝角，则实数λ的取值范围是 ______．
答案

∵向量

a
=(λ，2)，
b
=(-3，5)，且
a
，
b
的夹角是钝角，设两个向量的夹角为θ，

∴-1＜cosθ＜0，由两个向量的夹角公式得 cosθ=

a
•
b
|
a
•|
b
|
=
-3λ+10
λ2+4
×
9+25
=
10-3λ
λ2+4
×
34
，

∴-1＜

10-3λ
λ2+4
×
34
＜0，

∴λ＞

10
3
 且
34
×
λ2+4
＞3λ-10＞0，

∴λ＞

10
3
 且 （5λ+6）2＞0，

∴λ＞

10
3
，

故答案为 （

10
3
，+∞）．

单项选择题

Not everyone in the world requires the same amount of living space. The amount of space a person needs around him is a cultural difference, not an economic one. Knowing your own psychological space needs is important because they ply affect your choices, including, for example, the number of bedrooms in the home. If you were brought up in a two child family and both you and your sister or brother had your own bedrooms, the chances are if you have two children or more, that you also will offer separate bedrooms for them. In America, for example, they train people to want to have their own rooms by giving them their own rooms when they are babies. This is very unusual in the world. In many other countries, the baby sleeps in the same bed with his parents or in bed near them.
The space in the home also shows a lot about psychological space needs. Some families gather closer to each other and the size of their house has nothing to do with it. Others have separate little corners where family members go to be alone.
Although it is true that psychological space needs are not decided by economic reasons, they sometimes have to be changed a little because of economic pressure. It is almost impossible, however, to completely change your psychological space needs.

Some families gather closer to each other at home than others because ______.

A．they have limited living space
B．they are brought up in a large family
C．it satisfies their psychological space needs
D．the children in the family sleep in the same bed with their parents
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判断题

产生簇生叶序的主要原因是由于茎节间缩短的缘故。
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