题干

F(c, 0)是椭圆的右焦点,F与椭圆上点的距离的最大值为M,最小值为m,则椭圆上与F点的距离等于的点的坐标是                             (   )

A:(c, ±)

B:(-c, ±)

C:(0, ±b)

D:不存在

上一题 下一题 0.0难度 选择题 更新时间:2020-02-07 03:54:11

答案(点此获取答案解析)

C

同类题1

任务型阅读

    We all have our favourite teachers—those who treat us equally. But what about those teachers we don't know as well (for even don't like much)?

You can do lots of things to get a relation with your teacher. First,do the following:____ Complete all homework on time.Be attentive,be respectful,and ask questions.____ Obviously,your teachers are really interested in their subjects or they wouldn't have decided to teach them! Show the teacher that you care,even if you're not good at math or fluent in French—send the message that you are a dedicated student.

    You can also schedule a private meeting during a teacher's free period.____ You can ask questions,inquire about a career in the subject,or talk about your progress in class. You may be surprised to learn that your teacher is a bit more relaxed one on one than when lecturing in front of the whole class.

However,here are some things to avoid when trying to establish a relationship with your teacher:

    ⒈____ Teachers sense when your only motivation is to get special treatment,a college reference,or a job suggestion.

    ⒉Trying to be teachers' pet. You just attract the teachers' attention on purpose and your classmates may start to hate you.

    ⒊____ It's OK to offer a small sign of thanks to teachers if they've been helpful to you. But a teacher is usually not allowed to accept the wrong message,and a teacher is usually not allowed to accept anything expensive.

A.Giving expensive gifts.

B.Show up for class on time.

C.Not being sincere.

D.Find the practical value in classes.

E.Use this time to get extra help.

F.Show an interest in the subject.

G.Get along better with your teachers.

同类题2

现代文阅读
抗生素之殇
    ①1928年,英国微生物学家亚历山大·费莱明首次从青霉菌中发现了青霉素,从此人类进入了抗生素的黄金时代。第二次世界大战中,青霉素作为一线药用抗生素拯救了成千上万人的性命,价格曾一度比黄金还要昂贵。但在60多年后,很多抗生素失却了当年的雄风,成了医药学家的心头之痛。
    ②除了抗生素天然存在的毒副作用,抗生素耐药性问题是当今社会面临的重大问题之一。微生物适应环境的本领很强,当环境剧变时,它们不是束手待毙,而是用基因突变与环境抗争。生物与环境是统一的整体,成天生活在抗生素环境中的细菌,大部分不能存活,但总有漏网之鱼。侥幸存活的细菌在环境逼迫下会发生基因突变,一旦突变出抗药基因,就会绝处逢生。
    ③在大自然漫长的进化过程中,有“矛”就必有“盾”,抗生素固然是大自然产生的有效武器,但是大自然也必然保留着相应的破解方法。研究人员目前已经发现了数十类病原体具有耐受抗生素的抗性调控机制,这些病原体能够有效应对环境中出现的不同类型的抗生素。分裂生长速度越快的病原体,其耐药性产生的速度也就越快。
    ④据统计,人类使用抗生素之前和使用初期,世界范围内的耐药菌出现速度远慢于当今,随着不同种类抗生素的不断开发,临床上耐药菌开始呈指数级地出现。近年,多重耐药“超级细菌”的出现曾一度引起社会的恐慌,而目前新型抗生素的研发速度远慢于临床中耐药菌的产生速度。
    ⑤人们对于抗生素的滥用,正是在对耐药菌进行着无形的“人工筛选”。而如今世界范围内发达的交通运输使得这些人工筛选得到的耐药菌可以快速传播。即使你得了病坚持不用抗生素,体内确实没有筛选出大量的耐药菌,但是仍然有可能躺枪。
    ⑥虽然抗生素滥用的后果十分可怕,但是也不必杞人忧天和坐以待毙。首先,应该从每个人做起,不要感冒发烧后就自己给自己开药,应该遵循医嘱,科学合理用药。其次,应该呼吁医药和食品行业不要滥用抗生素,毫无节制地滥用抗生素无异于自掘坟墓。医药学家也正在与耐药菌赛跑,一方面改变药物的分子机构,以对付病原菌突变合成的酶;另一方面从世界各个生态环境中筛选新的抗生菌种。无论是赤日炎炎的热带,还是寒冷的极地;无论是大洋深处的喷口,还是地下的岩层,都有抗生素专家探求的目光。
(本文有删改)