题干

已知a为常数,函数f(x)=x(lnx﹣ax)有两个极值点x1,x2(x1<x2)(   )

A:f(x1)>0,f(x2)>12

B:f(x1)<0,f(x2)<12

C:f(x1)>0,f(x2)<12

D:f(x1)<0,f(x2)>12

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D

同类题1

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    While Jennifer was at home taking an online exam for her business law class, a monitor a few hundred miles away was watching her every move.

    Using a web camera equipped in Jennifer's Los Angeles apartment, the monitor in Phoenix tracked how frequently her eyes moved from the computer screen and listened for the secret sounds of a possible helper in the room. Her Internet access was locked-remotely-to prevent Internet searches, and her typing style was analyzed to make sure she was who she said she was: Did she enter her student number at the same speed as she had in the past? Or was she slowing down?

    In the battle against cheating,this is the cutting_edge and a key to encourage honesty in the booming field of online education. The technology gives trust to the entire system,to the institution and to online education in general. Only with solid measures against cheating, experts say, can Internet universities show that their exams and diplomas are valid—that students haven't just searched the Internet to get the right answers.

    Although online classes have existed for more than a decade, the concern over cheating has become sharper in the last year with the growth of “open online courses.” Private colleges, public universities and corporations are jumping into the online education field. Spending millions of dollars to attract potential students, while also taking steps to help guarantee honesty at a distance.

    Aside from the web cameras, a number of other high-tech methods are becoming increasingly popular. Among them are programs that check students' identities using personal information, such as the telephone numbers they once used.

    Other programs can produce unique exams by drawing on a large list of questions and can recognize possible cheaters by analyzing whether difficult test questions are answered at the same speed as easy ones. As in many university classes, term papers are scanned against some large Internet data banks for cheating.

同类题4

二氧化硫是重要的工业原料,探究其制备方法和性质具有非常重要的意义.

(1)工业上用黄铁矿(FeS2,其中S元素为﹣l价)在高温下和氧气反应制备SO2:4FeS2+11O2═8SO2+2Fe2O3,该反应中被氧化的元素是____ (填元素符号).当该反应转移2.75mol电子时,生成的二氧化硫在标准状况下的体积为____ L.

(2)实验室中用如图1所示的装置测定SO2催化氧化为SO3,的转化率.(已知SO3熔点为16.8℃,假设气体进入装置时分别被完全吸收,且忽略空气中CO2的影响.)

①简述使用分液漏斗向圆底烧瓶中滴加浓硫酸的操作是____ .

②实验过程中,需要通入氧气.试写出一个用图2所示装置制取氧气的化学方程式____ .

③当停止通入SO2,熄灭酒精灯后,需要继续通一段时间的氧气,其目的是____ .

④实验结束后,若装置D增加的质量为m g,装置E中产生白色沉淀的质量为ng,则此条件下二氧化硫的转化率是____ (用含字母的代数式表示,不用化简).

(3)某学习小组设计用如图3装置验证二氧化硫的化学性质.

①能说明二氧化硫具有氧化性的实验现象为____ .

②为验证二氧化硫的还原性,充分反应后,取试管b中的溶液分成三份,分别进行如下实验:

方案I:向第一份溶液中加入AgNO3溶液,有白色沉淀生成

方案Ⅱ:向第二份溶液加入品红溶液,红色褪去

方案Ⅲ:向第三份溶液加入BaCl2溶液,产生白色沉淀

上述方案中合理的是____ (填“I”、“Ⅱ”或“Ⅲ”);试管b中发生反应的离子方程式为____ .