There are certain set of general purpose operation which are quite frequently used by many programmers in their program. For example the square root of number to calculate power of a number and many more.
Making functions for performing these operations in every program is an unnecessary and time consuming job. So there general purpose operation are programmed and stored in C++ library so that they can be called through any program in the from of function.
These functions are called in Built function. There are number of library functions available in C++ .
Example
Program to find square root of number
#include <iostream.h>
#include <math.h>
int main()
{
int n, x;
cout,<"Enter any number";
cin >> n;
cout <<"square root of" <<x;
getch();
return0;
}