Menu.java

import java.util.Scanner;

public class Menu {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int chon;
        do {
            menu();
            System.out.print("Chon chuc nang: ");
            chon = sc.nextInt();
            switch (chon) {
                case 1:
                    giaiPTB1();
                    break;
                case 2:
                    giaiPTB2();
                    break;
                case 3:
                    tinhTienDien();
                    break;
                case 4:
                    System.out.println("Ket thuc chuong trinh.");
                    break;
                default:
                    System.out.println("Khong co chuc nang nay.");
                    break;
            }
        } while (chon != 4);
    }

    public static void menu() {
        System.out.println("+---------------------------------------------------+");
        System.out.println("1. Giai phuong trinh bac nhat");
        System.out.println("2. Giai phuong trinh bac hai");
        System.out.println("3. Tinh tien dien");
        System.out.println("4. Ket thuc");
        System.out.println("+---------------------------------------------------+");
    }

    public static void giaiPTB1() {
        Scanner sc = new Scanner(System.in);
        System.out.print("Nhap a: ");
        double a = sc.nextDouble();
        System.out.print("Nhap b: ");
        double b = sc.nextDouble();
        if (a == 0) {
            if (b == 0) {
                System.out.println("Phuong trinh vo so nghiem.");
            } else {
                System.out.println("Phuong trinh vo nghiem.");
            }
        } else {
            double x = -b / a;
            System.out.println("Nghiem cua phuong trinh la: x = " + x);
        }
    }

    public static void giaiPTB2() {
        Scanner sc = new Scanner(System.in);
        System.out.print("Nhap a: ");
        double a = sc.nextDouble();
        System.out.print("Nhap b: ");
        double b = sc.nextDouble();
        System.out.print("Nhap c: ");
        double c = sc.nextDouble();
        if (a == 0) {
            System.out.println("Day khong phai phuong trinh bac hai.");
            return;
        }
        double delta = b * b - 4 * a * c;
        if (delta < 0) {
            System.out.println("Phuong trinh vo nghiem.");
        } else if (delta == 0) {
            double x = -b / (2 * a);
            System.out.println("Phuong trinh co nghiem kep: x = " + x);
        } else {
            double x1 = (-b + Math.sqrt(delta)) / (2 * a);
            double x2 = (-b - Math.sqrt(delta)) / (2 * a);
            System.out.println("Phuong trinh co 2 nghiem phan biet:");
            System.out.println("x1 = " + x1);
            System.out.println("x2 = " + x2);
        }
    }

    public static void tinhTienDien() {
        Scanner sc = new Scanner(System.in);
        System.out.print("Nhap so dien su dung trong thang: ");
        int soDien = sc.nextInt();
        int tien;
        if (soDien <= 50) {
            tien = soDien * 1000;
        } else {
            tien = 50 * 1000 + (soDien - 50) * 1200;
        }
        System.out.println("So tien phai tra: " + tien + " VND");
    }
}