acos

    double acos(double);

 calculates arccosine

#include <iostream> /* including standard library */
#include <cmath> /* including math library */
 
using namespace std;
int main ( void ){
    double a=0.2, res=0;
    res = acos(a) * 90 / 3.17;
 
    //cout.setf(ios::fixed, ios::floatfield);
    cout.precision(8);
 
    cout << "acos from " << a << " is " << res << endl;
    return 0;
}

  acos from 0.2 is 38.879955