题目

若实数、满足，则的取值范围是（ ）A. B. C. D. 答案：【答案】D【解析】根据不等式组作出可行域，根据的几何意义：可行域内的点与原点连线的斜率，据此计算出的取值范围.作出可行域如下图：由图可知：当点在直线上时，此时斜率最小为：，当点靠近轴上，此时斜率，所以.故选：D.Used by over half a million learners，English Practice is one of the world’s most popular online English learning products. English Practice offers over 40，000 interactive 1essons for all levels of students who want to study and improve their English. Our structured content will help you improve your speaking，vocabulary，grammar，and your listening and written comprehension. And because English Practice is online，you can study at a pace and at a time that is convenient for you. How to StudyWhen you start with English Practice we will give you a test to evaluate your language level. We will then recommend that you start as either a beginning intermediate or advanced learner. At each level you can take classes in General English，Work Skills，or Community to improve the skills you need. When you have completed enough lessons，we will give you another test and move you up to the next level. General EnglishThis is where you will find lessons to improve your grammar，listening comprehension，and vocabulary. There are new lessons weekly so you will always find new 1essons to this area you can also take lessons to prepare for TOEFL and IELTS. Work SkillsThis is where you can take lessons to teach you the specific skills you will need if you have to speak English at work，or do business in English. There are lessons on general business skills as well as courses to on specific industries. CommunityIn the Community section you find English Practice’s popular chat room，where you can practice with fellow students from around the world，and ask English teachers questions about English Community also has fun games like Bingo，Hangman，and Karaoke. As well as links to online English studying resources. 65. Compared with the courses in class，English practice has the advantage that        . A. you can improve your English quickly through itB. it will offer you the method of how to study English wellC. you can plan your time for English study according to your practical situationD. you can teach yourself English at any time66. If you want to study English breezily（轻松地），you should get into the section. A. General English                                            B. Work SkillsC. Community                                                 D. How to study67. If you are a student and want to add to your basic knowledge of English，you’d  better        . A. take the test offered by English PracticeB. take the General EnglishC. take the classes in Work SkillsD. the classes in Community68. People can most probably read this article. A. on the Internet                                             B. in an instruction bookC. in a magazine                                               D. in a newspaper