Instead, ``#include'' lines are special instructions for the C compiler. They tell the compiler about other functions that we are invoking in our program.
Take a look at our sample C program again. The line ``#include <stdio.h>'' tells the compiler that we are going to use the functions that do input and output. The abbreviation stdio stands for ``standard input/output.''
The line ``#include <math.h>'' tells the compiler that we are going to use some math functions, such as sine and cosine. (In C those functions are called sin and cos.)
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