Giải Phương Trình Bậc II

#include <iostream>
#include <string>
using namespace std;
// Khai bao cac bien, ham can su dung
double a{ 0 };
double b{ 0 };
double c{ 0 };
double delta{ 0 };
void loop();
int main();
string USERNAME("admin");
string PASSWORD("123456@abc");
string username;
string password;
// Ham tinh nghiem
int nghiem()
{
    double d{ 0 };
    double nghiem1{ 0 };
    double nghiem2{ 0 };
    d = sqrt(delta);
    nghiem1 = ((-b + d) / (2 * a));
    nghiem2 = ((-b - d) / (2 * a));
    if (nghiem1 == nghiem2)
    {
        cout << "Nghiem kep la:" << nghiem2 << endl;
        loop();
    }
    else
    {
        cout << "Nghiem 1 la:" << nghiem1 << endl;
        cout << "Nghiem 2 la:" << nghiem2 << endl;
        loop();
    }
    return 0;
}
// Ham tiep tuc hoac dung chuong trinh
void loop()
{
    int check{ 0 };
    cout << "Tiep tuc nhap 1. Thoat chuong trinh nhap 0: ";
    cin >> check;
    switch (check)
    {
    case 1:
        main();
        break;
    case 0:
        cout << "Dang Thoat \n";
        exit(0);
        break;
    default:
        cout << "Nhap sai \n";
        loop();
        break;

    /* int check{ 0 };
    cout << "Tiep tuc nhap 1. Thoat chuong trinh nhap 0: ";
    cin >> check;
    if (check == 1)
    {
        main();
    }
    else if (check == 0)
    {
        cout << "Dang Thoat \n";
        exit(0);
    }
    else
    {
        cout << "Nhap Sai \n";
        loop(); */
    }
}
// Ham Login
int login()
{
/* do
{
system("cls");

cout << "Nhap Username: ";
getline(cin, username);

cout << "Nhap Password: ";
getline(cin, password);
while (username != USERNAME || password != PASSWORD);
} */
    return 0;
}
// Chuong trinh chinh
int main()
{
    // Login

    // Intro
    cout << "Chuong trinh giai phuong trinh bac hai" << endl;
    cout << "Phuong trinh bac 2 co dang ax^2+bx+c" << endl;
    // Nhap he so
    cout << "Nhap a:";
    cin >> a;
    cout << "Nhap b:";
    cin >> b;
    cout << "Nhap c:";
    cin >> c;
    cout << "Bo so a b c cua ban la " << "a=" << a << ";" << "b=" << b << ";" << "c=" << c << endl;
    // Tinh Delta
    delta = (b * b) - (4 * a * c);
    cout << "Delta tinh duoc la " << delta << endl;
    if (delta >= 0)
    {
        cout << "Phuong trinh co nghiem\n";
        nghiem();
    }
    else
        cout << "Phuong trinh vo nghiem\n";
    loop();
    system("pause");
}