3) uses crt;
var i : integer;
 st : string;
begin clrscr;
write('Nhap xau :'); readln(st);
for i:=length(st) downto 1 do write(st[i]);
readln end.

1) và 2) :

uses crt;
var a,b : integer;
begin clrscr;
assign(input,'songuyen.txt2');
assign(output,'songuyen.txt3');
reset(input); rewrite(output);
read(input,a,b);
write(output,a+b);
close(input); close(output);
readln
end.

- Chương trình 1 :

uses crt;

var a, b : integer; f : text;

begin

clrscr;

write('Nhap hai so a, b = '); readln(a, b);

assign(f, 'songuyen.txt 2');

rewrite(f); write(f, a, ' ', b);

close(f);

end.

- Chương trình 2 :

uses crt;

var a, b : integer; f : text;

begin

clrscr;

assign(f, 'songuyen.txt 2'); reset(f);

read(f, a, b);

write('Tong hai so = ', a + b);

readln

end.

- Chương trình 3 :

uses crt;

var S : string; i : byte;

begin

clrscr;

write('Nhap xau : '); readln(S);

write('Xau dao : ');

for i := length(S) downto 1 do write(S[i]);

readln

end.