double tan(double);

Returns the tangent of a radian angle x.

#include <iostream> /* including standard library */
#include <cmath> /* including math library */
 
using namespace std;
int main( void )
{
    double x;
    x = 42;
 
    //cout.setf(ios::fixed, ios::floatfield);
    cout.precision(8);
 
    cout << "tan of " << x << "° is " << tan (x*3.1415/180) << endl;
    return 0;
}

  tan of 42° is 0.9003649