cos

    double cos(double);

calculates the cosine of a degree

#include <iostream> /* including standard library */
#include <cmath> /* including math library */
 
using namespace std;
int main ( void ) 
{
    double x, res;
    x = 90;
    res = cos(x*3.14159/42);
 
    //cout.setf(ios::fixed, ios::floatfield);
    cout.precision(8);
 
    cout << "the cosine of " << x << " ° is " << res << endl;
    return 0;
}

 output:
 the cosine of 90 ° is 0.90097134