C++ code for Simpson's 1/3 Rule

This is the solution for finding Integration using Simpson's 1/3 Rule

#include <iostream>
#include <math.h>

using namespace std;


class Simpsons1b3
{
public:
    Simpsons1b3()
    {
        k = 2;
    }
    void askInterval();
    void solve();
    double f(double x);
    void findH();
    void findX1();
private:
    double I,h,xn,x0,k,x1;

};

void Simpsons1b3::askInterval()
{

    cout << "Enter Xn: ";
    cin >> xn;
    cout << "Enter X0: ";
    cin >> x0;
}

void Simpsons1b3::findH()
{
    h= (xn - x0)/k;
}

void Simpsons1b3::findX1()
{

    x1 = x0 + h;
}


double Simpsons1b3::f(double x)
{
    return exp(x);
}

void Simpsons1b3::solve()
{
    askInterval();
    findH();
    findX1();
    I = h / 3.0 * ( f(x0) +4 * f(x1) + f(xn) );
    cout << endl << "The required answer is : " << I << endl;
}
int main()
{
    Simpsons1b3 s1;
    s1.solve();
    return 0;
}