This course is for those who want to learn to type, as well as those who want to improve their typing. The course is not common. You are tested in the first class, and begin practising at one of eight different skill levels. This allows you to learn at your own speed. Each program lasts 20 hours. Bring your own paper.
Course fee: $125      Materials:  $25
     Two hours for each evening for two weeks. New classes begin every two weeks.
This course is taught by a number of professional business education teachers who have successfully taught typing course before.
UNDERSTANDING COMPUTERS
This twelve-hour course is for people who do not know very much about computers, but who need to learn about them. You will learn what computers are, what they can or can't do and how to use them.
Course fee：$75  Jan 4,7,11,14,18  Wed.&Sat. 9:00---11:30 a.m.  Equipment fee:$10
David is a professor of Computer Science at Beijing University. He has over twenty years of experience in the computer field.  
STOP SMOKING
    Do you want to stop smoking? Have you ever tried and failed? Now is the time to stop smoking using the latest methods. You can stop smoking, and this twelve-course will help you do it.
Course fee：$30  Jan 2,9,16,23   Mon 2:00---5:00 p.m.
Dr. John is a practicing psychologist who has helped hundreds of people stop smoking.

在边长为a的等边三角形ABC中，AD⊥BC于D，沿AD折成二面角B﹣AD﹣C后，BC=$\frac{a}{2}$，这时二面角B﹣AD﹣C的大小为____ ．

三四____

已知椭圆$\frac{x^2}{a^2}+\frac{y^2}{b^2}=1\left(a>b>0\right)$的离心率$e=\frac{1}{2}$，右焦点为$F\left(c,0\right)$，方程$a{x}^2+bx-c=0$的两个实根$x_1,x_2$，则点$P\left(x_1,x_2\right)$（   ）

如果三角形的三边长分别为4、6、8，那么连结该三角形三边中点所得三角形的周长是（　　）