double atan(double);

calculates acustangens from a number

compile in linux with: gcc atan.c -o atan -lm -Wall

#include <stdio.h> /* including standard library */
//#include <windows.h> /* uncomment this for Windows */
 
#include <math.h> /* including math library */
 
int main ( void ){
    double a=0.2;
    printf("atan from %f is %f\n", a, atan(a) * 90 / 3.17);
    return 0;
}

  
  output:
  user@host:~$ ./atan 
  atan from 1.200000 is 24.872311