截棱长为1的正方体
,所得的截面的周长记为
,则当平面
经过正方体的某条体对角线时,
的最小值为( )
B .
C .
D .

物质 | 分子的价层电子对数 | 中心原子的杂化轨道类型 | 分子的立体构型名称 |
X2Z | V形 | ||
YZ2 |
.
时,判断
是否为
的极值点,并说明理由;
.若函数
存在极大值
,证明:
.
做空气中氧气含量测定的实验装置如图。下列有关说法正确的是( )
如图,∠AOC>∠BOD,则( )


Mr Smith is an English teacher. He comes from America. He teaches English in China. He speaks Chinese quite well. He reads a lot of books and his wife often writes books for the students at home. Mr Smith gets up at five in the morning. He leaves home at ten to six. He begins the first lesson at eight. He teaches English every day except(除了) Saturday and Sunday. Usually at ten to twelve he finishes his lessons. Ten minutes later he has lunch. In the afternoon, he often plays games with his students. Sometimes he teaches his students in his office. Supper time is at about six. After that, he reads newspapers and watches TV. At about ten to eleven he goes to bed.
①可以促进与他人交往
②有利于增强对自己的信心
③可以促进自我发展
④可以使自己成为十全十美的人
的解为非负数,则m的取值范围是( )
B .
且
C .
D .
且