Cách 1: 

var a,b,UCLN,BCNN:integer;
begin
       write('Nhap vao 2 so a va b');
       readln(a,b);
       BCNN:=a*b;
       While a<>b do If a>b then a:=a-b else b:=b-a;
       UCLN:=a;
       BCNN:=BCNN div UCLN;
       write(UCLN,' ',BCNN);
end.

Cách 2:Thuật toán Euclide

var a,b,UCLN, BCNN, temp:integer; {temp là biến tạm}
begin
      write('Nhap vao so a: ');
      readln(a);
      write('Nhap vao so b: ');
      readln(b);
      BCNN:= a*b;
      temp:= b mod a;
      while temp<>0 do
            begin
               temp:= a mod b;
               a:=b;
               b:=temp;
             end;
      UCLN:=a;
      BCNN:= BCNN div UCLN;
      write('UCLN = ',UCLN,' BCNN = ',BCNN);
      readln;
end.

program ct;

uses crt;

var a,b,UCLN:longint;

begin clrscr;

   write('Nhap 2 so a va b: '); readln(a,b);

   while a<>b do

     if a>b then a:=a-b

                else b:=b-a;

     UCLN:=a;

   writeln('Uoc chung lon nhat cua hai so la: ',UCLN);

readln;

end.

Học tốt!