Trapezoidal rule

#include <iostream>

using namespace std;

typedef double (* function) (double x);

double f(double x){
    return x;
}

double X(int i, double a, double h){
    return a + i * h;
}

double Trapezoidal(double a, double b, int N, function f){
    double h = (b - a) / double(N);

    double S = 0;
    for(int i = 1; i <= N; ++i){
        S += h * (f(X(i - 1, a, h)) + f(X(i, a, h))) / 2.;
    }

    return S;
}

int main(){
    double a, b;
    cout << "Enter a, b:\n";
    cin >> a >> b;

    cout << "Enter the number of segments:\n";
    int N;
    cin >> N;

    cout << Trapezoidal(a, b, N, f);

    system("PAUSE");
    return 0;
}