C库函数 acos()

#include <stdio.h>

#include <math.h>



#define PI 3.14159265



int main () {

   double x, ret, val;



   x = 0.9;

   val = 180.0 / PI;



   ret = acos(x) * val;

   printf("The arc cosine of %lf is %lf degrees", x, ret);



   return(0);

}