若函数f（x）=﹣x•ex，则下列命题正确的是_刷题宝

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若函数f（x）=﹣x•e^x，则下列命题正确的是（）

A：∀a∈（﹣∞， $\frac{1}{e}$ ），∃x∈R，f（x）＞a

B：∀a∈（ $\frac{1}{e}$ ，+∞），∃x∈R，f（x）＞a

C：∀x∈R，∃a∈（﹣∞， $\frac{1}{e}$ ），f（x）＞a

D：∀x∈R，∃a∈（ $\frac{1}{e}$ ，+∞），f（x）＞a

上一题下一题 0.0难度选择题更新时间:2019-03-07 03:43:06

答案(点此获取答案解析）

A

同类题1

\sqrt{2}

\sqrt{2}

同类题2

人们对伽利略的理想实验的认识不正确的是（　　）

同类题3

阅读下面短文，从每小题A、B、C、D四个选项中，选出能回答所提问题的最佳答案。

C

In America, boys and girls often join clubs. The members of a club work together. They also learn certain skills and share good times.

Many boys belong to Boy Scouts. Boy scouts enjoy hiking and camping out together, and they learn many things. Scouts learn safety rules, first aid, and what to do when in danger. Members have a chance to learn outdoor living. They learn to build fires and cook out in the open.

Girl Scouts and Camp Fire Girls are clubs for girls. These two clubs give their members a chance to have fun and learn. The girls in a group take trips, have parties and go hiking and camping together. The Camp Fire Girls earn honor beads( 荣誉珠) for learning such skills as swimming and cooking. The Girl Scouts earn badges(徽章).

Both boys and girls can be members of 4-h clubs, Children who live in either the city or the country may belong to 4-h. A project such as raising a pig or a calf can be carried on only by farm children, while city children can take part in gardening and cooking projects.

In all these clubs the boys and girls learn to be good and useful citizens.

同类题4

下列不能说明氯元素的非金属性比硫元素强的事实是（）

①HCl比H₂S稳定 ②HClO氧化性比H₂SO₄强 ③HClO₄酸性比H₂SO₄强

④Cl₂能与H₂S反应生成S ⑤Cl原子最外层有7个电子，S原子最外层有6个电子

⑥Cl₂与Fe反应生成FeCl₃，S与Fe 反应生成FeS．

同类题5

图为一放射源发出的α、β、γ射线进入同一匀强磁场（磁场未画出）中的径迹．下表列出了三种射线的本质和特性．

种类	本质	质量（u）	电荷（e）	速度（c）
α射线	氦核	4	+2	0.1
β射线	电子	$\frac{1}{1840}$	﹣1	0.99
γ射线	光子	0	0	1

由此可知，（）

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