Predefined Functions

The C/C++ libraries have many predefined functions you can use in your program.
You generally need to include one or more header files (with the #include directive) to use these functions.
The <cmath> header includes the declaration of the sqrt function used in the example and the pow function that computes powers of a value.
A function must be declared and defined in order to use it.
The sqrt function is declared in the <cmath> header and it is defined in the library.