问题 选择题

给出三个命题:

①若两条直线和第三条直线所成的角相等,则这两条直线互相平行;

②若两条直线都与第三条直线垂直,则这两条直线互相平行;

③若两条直线都与第三条直线平行,则这两条直线互相平行。其中真命题个数是(   )

A.0

B.1

C.2

D.3

答案

答案:B

题目分析:对于①,两条直线和第三条直线所成角相等,以正方体ADCD-A1B1C1D1为例,过点A的三条棱AA1、AB、AD当中,AB、AD与AA1所成的角相等,都等于90°,但AB、AD不平行,故①错误;

对于②,两条直线与第三条直线都垂直,以正方体ADCD-A1B1C1D1为例,过点A的三条棱AA1、AB、AD当中,两条直线AB、AD都与AA1垂直,但AB、AD不平行,故②错误;

对于③,若直线a、b、c满足a∥b且b∥c根据立体几何公理4,可得a∥c,说明两条直线都与第三条直线平行,则这两条直线互相平行。故③是正确的.综上所述,不正确的为①②,故选B。

点评:本题以命题真假的判断为载体,考查了一些在平面内成立的命题推广到空间能否为真命题等知识点,属于基础题.

单项选择题

Motherhood may make women smarter and may help prevent dementia in old age by bathing the brain in protective hormones, US reseachers reported on Thursday.
Tests on rats show that those who raise two or more litters of pups do considerably better in tests of memory and skills than rats who have no babies, and their brains show changes that suggest they may be protected against diseases such as Alzheimer’s. University of Richmond psychology professor Craig Kinsley believes his findings will translate into humans.
"Our research shows that the hormones of pregnancy are protecting the brain, including estrogen, which we know has many neuroprotective effects, " Kinsley said.
"It’s rat data but humans are mammals just like these animals are mammals, " he added in a telephone interview. "They go through pregnancy and hormonal changes. "
Kinsley said he hoped public health officials and researchers will look to see if having had children protects a woman from Alzheimer’s and other forms of age-related brain decline.
"When people think about pregnancy, they think about what happens to babies and the mother from the neck down, " said Kinsley, who presented his findings to the annual meeting of the Society of Neuroscience in Orlando, Florida.
"They do not realize that hormones are washing on the brain. If you look at female animals who have never gone through pregnancy, they act differently toward young. But if she goes through pregnancy, she will sacrifice her life for her infant—that is a great change in her behavior that showed in genetic alterations to the brain. \

What does the phrase "litters of pups" mean in the second paragraph

A. Baby rats.


B. Animals.
C. Old rats.



D. Grown-up rats.

问答题 简答题