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电阻R1=3Ω，R2=7Ω，把它们串联接入电路时，两端电压之比为U1：U2=______；若把这两个电阻并联接入电路时，两端电流之比为I1：I2=______．

答案

（1）两电阻串联时，通过它们的电流相等，

由欧姆定律可得：

U1
U2
=
IR1
IR2
=
R1
R2
=
3Ω
7Ω
=
3
7
；

（2）两电阻并联时，电阻两端的电压相等，则

I1
I2
=
U
R1
U
R2
=
R2
R1
=
7Ω
3Ω
=
7
3
．

故答案为：3：7；7：3．

选择题
已知随机变量ξ～N（0，σ2）～ξ～N（0，σ2），若P（-2≤ξ≤0）=0.2，则P（ξ≥2）等于（　　）
A．0.1B．0.2C．0.3D．0.4
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What is distance learning It means that you study on your own, at home or wherever suits you. Recently, the world famous Open University in the United Kingdom has designed a new style of distance learning, which is called (1) . The phrase "Open Learning" means you study (2) . You read course material, work on course activities, and write (3) . The word "Supported" means you have help (4) , the student services staff at regional centres, and centralized areas such as (5) . You can also contact other students through tutorials and (6) , the University’s online conferencing system, and events and clubs organized by (7) . Most distance learning courses use printed paper materials. They also include some (8) materials such as a CD, DVD or video. Many courses have a web site and an (9) . You’ll need access to a computer (10) to make use of these. The Open University can help its students buy a computer and (11) the cost of accessing the Internet.

With most distance learning courses, no (12) are required to study. of course, you have to be aged 18 when your course starts but there is no (13) . Currently the Open University has around (14) undergraduate and more than 30,000 postgraduate students, of which 10,000 have (15) . Nearly all students are studying (16) . About 70 per cent of undergraduate students are in (17) . More than 50,000 students (18) by their employers for their studies. Most distance learning courses (19) . Some of them are even available in other parts of the world. With over 25,000 of its students living outside the UK, the Open University is the (20) that offers distance learning throughout the world.

14（）
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