Program UCLN;
uses crt;
var a,b,r:byte;
begin
 clrscr;
 writeln('CHUONG TRINH TIM UCLN CUA HAI SO');
 write('Nhap a: ');readln(a);
 write('Nhap b: ');readln(b);
 r:=a mod b;
 while r<> 0 do
 begin
 b:=r;
 a:=b;
 r:=a mod b;
 end;
 write('UCLN cua hai so la: ',b);
 readln
end.

chúc bn hc tốt # nguyễn thanh bảo kha

While do

program bai_giai;
uses crt;
var a,b:integer;
begin
    clrscr;
    readln(a,b);
    while a<>b do
        if a>b then a:=a-b else b:=b-a;
    write(a);
    readln;
end.

for do, if then

program bai_giai;
uses crt;
var a,b,i,ucln,min:integer;
begin
    clrscr;
    readln(a,b);
    ucln:=1;
    if a>b then min:=b else min:=a;
    for i:=2 to min do
        if (a mod i=0) and (b mod i=0) then ucln:=i;
    write(ucln);
    readln;
end.