Inegration omp

// integral_sequential.cpp : Defines the entry point for the console application.
//
#include "stdafx.h"
#include <iostream>
#include <cmath>
#include <ctime>
#include <omp.h>

using namespace std;

double f(double x);

double rectangle_method(double a, double b, int n) {
    double sum = 0;
    double h = (b - a) / n;
    int i;
    double cur = 0 , x;
    omp_set_num_threads(4);
#pragma omp parallel for reduction(+:sum)
    for (i = 0; i < n; ++i) {
        x = a + h * i;
        sum = sum + f(0.5 * (x + x + h));
    }

    return sum * h;
}

double rec_method_seq(double a, double b, int n) {
    double h = (b - a) / n;
    double sum = 0.0;
    for (int i = 0; i < n; ++i) {
        double x = a + h * (double)i;
        sum += f(0.5 * (x + x + h));
    }
    return h * sum;
}

double f(double x) {
    return sin(x);
}

inline void solve(double a, double b, int n) {
    long long l = clock();
    double ans = rectangle_method(a, b, n);
    cout << "Parallel I(x) = " << ans << endl;
    cout << "Parallel time: " << (double)(clock() - l) / CLOCKS_PER_SEC << endl << endl;
}

inline void solve_seq(double a, double b, int n) {
    long long l = clock();
    double ans = rec_method_seq(a, b, n);
    cout << "Sequential I(x) = " << ans << endl;
    cout << "Sequential time: " << (double)(clock() - l) / CLOCKS_PER_SEC << endl << endl;
}

int main(int argc, char** argv)
{
    cout.precision(5);
    cout.setf(ios::fixed);
    double a, b, n;
    a = 0;
    b = 10000;
    n = 1e9;

    solve(a, b, n);

    solve_seq(a, b, n);//sequentival solve


    system("pause");
    return 0;
}