# include & ltiostream & gt is an include command, and its function is to include the contents of the file iostream into the program command where the command is located, not the command line. The function of iostream file is to provide the program with some information needed for input or output. I-o-stream is a combination of I-o-streams. From its form, we know that it stands for iostream.

Use Namespace std Use Namespace std, and classes and functions in the C++ standard library are declared in the namespace std, so this line is needed if the standard library is used.

# include & ltcmath & gt Add this line when using mathematical functions, that is, cmath is a mathematical function file, which contains mathematical functions, so that mathematical functions can be used.

For beginners, you only need to know that if there is a program input or output, you must use # include.

You can also buy some reference books. .

I wish you a happy study.