搜题请搜索小程序“爱下问搜题”
问题 选择题
若两个正数的算术平均数（即两数和的一半）为2
3
，几何平均数（即两数积的算术平方根）为
3
，则这两个数的差是（　　）
A．±6B．6C．±36D．36
答案

设这两个数分别为a和b，

则根据题意得：

a+b
2
=2
3
  ①，
ab
=
3
  ②，

①②两式联立解得：a2+b2=（a+b）2-2ab=48-6=42，

∴（a-b）2=a2+b2-2ab=42-2×3=36，

∴a-b=±6．

故选A．

单项选择题

About fifty years ago, plant physiologists set out to grow roots by themselves in solutions in laboratory flasks. The scientists found that the nutrition of isolated roots was quite simple. They required sugar and the usual minerals and vitamins. However, they did not require organic nitrogen compounds. These roots got along fine on mineral inorganic nitrogen. Roots are capable of making their own proteins and other organic compounds. These activities by roots require energy, of course. The process of respiration uses sugar to make the high energy compound ATP, which drives the biochemical reactions. Respiration also requires oxygen. Highly active roots require a good deal of oxygen.The study of isolated roots has provided an understanding of the relationship between shoots and roots in intact plants. The leaves of the shoots provide the roots with sugar and vitamins, and the roots provide the shoots with water and minerals. In addition, roots can provide the shoots with organic nitrogen compounds. This comes in handy for the growth of buds in the early spring when leaves are not yet functioning. Once leaves begin photosynthesizing, they produce protein, but only mature leaves can "export" protein to the rest of the plant in the form of amino acids.

It can be inferred from the passage that, in the early spring, the buds of plants________.

A．export protein in the form of amino acids

B．do not require water

C．have begun photosynthesizing

D．obtain organic compounds from the root
查看答案
问答题 论述题

2014年10月24日02时00分，我国自行研制的探月工程三期再入返回飞行试验器，在西昌卫星发射中心用长征三号丙运载火箭发射升空，准确进入预定轨道。我国探月工程首次实施的再入返回飞行试验首战告捷。此次任务是我国探月工程三期一次重要的验证飞行试验，从近期看，主要目的是突破和掌握探月航天器再入返回的关键技术，为嫦娥五号任务提供技术支持。从长远看可以为提高我国深空探测能力、利用月球资源为人类服务做准备！当然，探月工程的每一次进步，都离不开探月人不怕困难奋力拼搏的舍小家顾大家的奉献精神！

请运用意识的能动作用的知识分析上述材料。
查看答案