"vertical angles 对顶角
right angle直角
straight angle平角(180度)
A polygon is a closed figure formed by the intersection of three or more line segments, called sides,..."
"Whenever an integer N is divisible by an integer x, we say that x is a divisor of N.
For the set of positive integers, any integer N that has exactly two dis..."
"如果用户对某一个命令详细的了解的话，可以使用Man命令。例如，想了解ls命令的详细信息，那么命令行书写格式如下：［root @teacher root］# man ls注：在Linux中命令是区分大小写的，如：Ls 跟 ls 是不一样的。
pwd 命令代表“print working directory”（打印工作..."
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