C语言库atan()函数

#include <stdio.h>
#include <math.h>

#define PI 3.14159265

int main () {
   double x, ret, val;
   x = 1.0;
   val = 180.0 / PI;

   ret = atan (x) * val;
   printf("The arc tangent of %lf is %lf degrees", x, ret);

   return(0);
}