testing dowhile

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package lab2_mob1013_ps05854_nguyendttam;
import java.util.Scanner;

public class Lab2_ps05854_bai1 {

    public static void main (String args [])
    {

        Scanner sc=new Scanner(System.in);
    do {
    try
            {
        System.out.println("     Xin chao den voi LAB 2    ");
        System.out.println("________________________________");
        System.out.println("|             MENU             |");
        System.out.println("| 1. Giai phuong trinh bac nhat|");
        System.out.println("| 2. Giai phuong trinh bac 2   |");
        System.out.println("| 3. Tinh tien dien            |");
        System.out.println("| 4. Ket thuc                  |");
        System.out.println("________________________________");
        System.out.print("Xin moi chon chuc nang: ");
        int a=sc.nextInt();
        if(a>5){System.out.println("Chi duoc nhap so tu 1 den 4");}

            switch(a)
            {
                case 1:
                {
                    Bai1();
                continue;
                }
                case 2:
                {
                    Bai2();
                continue;
                }
                case 3:
                {
                    Bai3();
        continue;
                }

                case 4: {
            System.out.print("Ban da thoat khoi chuong trinh");
            break;
        }
            }
        }catch (Exception ex)
        {
        System.out.print("Xin kiem tra lai");
        }
    }while(true);

    }

    public static void Bai1 () {
       try
       {
        System.out.println("---Giai phuong trinh bac 1---");
           Scanner sc=new Scanner(System.in);

       System.out.print("Nhap he so a: ");
       int a=sc.nextInt();
       System.out.print("Nhap he so b: ");
       int b=sc.nextInt();
        if(a==0)
        {
            if(b==0)
            {
                System.out.print("Phuong trinh co vo so nghiem! ");
            }
            else
            {System.out.print("Phuong trinh vo nghiem!");}
        }
        else
        {System.out.print("Phuong trinh co nghiem la: " + (-b/a));}
    }
       catch(Exception ex)
               {
                   System.out.print("!!!Xin kiem tra lai he so da nhap!!!");
               }


    }
    public static void Bai2()
    {

        try
        {
            Scanner sc=new Scanner(System.in);
            System.out.println("---Giai phuong trinh bac hai---");
            System.out.print("Nhap he so a: ");
            int a=sc.nextInt();
            System.out.print("Nhap he so b: ");
            int b=sc.nextInt();
            System.out.print("Nhap he so c: ");
            int c=sc.nextInt();
            switch(a)
            {
                case 0:
                {if(a==0)
                    {
                        if(b==0)
                        {System.out.print("Phuong trinh co vo so nghiem! ");}
                        else{System.out.print("Phuong trinh vo nghiem!");}
                    }
                  else{System.out.print("Phuong trinh co nghiem la: " + (-b/a));}
                }
                break;
            default:
            {
                double Delta=Math.pow(b,2)-4*a*c;
                if (Delta<0){System.out.print("Phuong trinh vo nghiem");}
                else if (Delta==0)
                {System.out.print("Phuong trinh co nghiep kep x la: " + (-b/(2*a)));}
                else
                {
                System.out.println("Phuong trinh co 2 nghiem la:");
                System.out.print("x1=" + ((-b+Math.sqrt(Delta))/(2*a)));
                System.out.print("x2=" + ((-b-Math.sqrt(Delta))/(2*a)));
                }
                break;
            }
            }

        }   catch(Exception ex)
               {
                   System.out.print("!!!Xin kiem tra lai he so da nhap!!!");
               }

            }

     public static void Bai3()
       {
           try
           {
              System.out.println("---Tinh tien dien---");
               Scanner sc=new Scanner(System.in);
               System.out.print("Nhap vao so dien: ");
               int soDien=sc.nextInt();
               if(soDien<50)
               {
                 System.out.print("So tien phai tra la: "+soDien*1000);
               }
               else{System.out.print("So tien phai tra la: "+soDien*1200);}
           }
           catch(Exception ex)
           {
           System.out.print("!!!Xin kiem tra lai!!!");
           }
       }
}