1. 

program phuongt;

uses crt;

  vaar a, b, c, dt: integer;

begin

         clrscr;

         write('Nhap a = '); readln(a);

         write('Nhap b = '); readln(b);

         write('Nhap c = '); readln(c);

         dt:=0;

         if (a = 0) and (b = 0) and (c = 0) then writeln('Phuong trinh co vo so nghiem');

         if (a = 0) and (b = 0) and (c <> 0) then writeln('Phuong trinh vo nghiem');

         if (a = 0) and (b <> 0) then writeln('Phuong trinh co nghiem: ', -c/b:2:1);

         if (a <> 0) then

         begin

                   dt:=b*b-4*a*c;

                   if dt<0 then writeln('Phuong trinh vo nghiem');

                   if dt=0 then writeln('Phuong trinh co nghiem kep la: x1 = x2 = ',-b/(2*a):2:1);

                   if dt>0 then writeln('Phuong trinh co 2 nghiem: x1 = ', (-b-sqrt(a))/(2*a):2:1, '; x2 = ', (-b-sqrt(dt))/(2*a):2:1);

         end;

         readln

end.

2.

program phuongt;

uses crt;

  var a, b: integer;

begin

         clrscr;

         write('Nhap a = '); readln(a);

         write('Nhap b = '); readln(b);

         if (a = 0) and (b = 0) then writeln('Phuong trinh co vo so nghiem');

         if (a = 0) and (b <> 0) then writeln('Phuong trinh vo nghiem');

         if a <>0 then writeln('Phuong trinh co nghiem: x = ', -b/a:2:1);

         readln

end.

3.

program phuongt;

uses crt;

  var n, i, d: integer;

begin

         clrscr;

         write('Nhap n ='); readln(n);

         d:=0;

         if n>=2 then

         begin

                   for i:=2 to trunc(n/2) do if n mod 1 = 0 then d:=d+1;

                   if d=0 then write(n, ' la so nguyen to') 

                   else write(n, ' khong phai la so nguyen to');

         end

         else write('Nhap sai!');

         readln

end.