Cách 1 :

uses crt;

var i, s, n : integer;

begin

clrscr;

write('nhap so n :');readln(n);

s := 0;

for i := 1 to n do

s := s + i;

writeln('ket qua s :', s);

readln

end.

Cách 2 :

uses crt;

var i, s, n : integer;

begin

clrscr;

write('nhap so n :');readln(n);

s := 0;

for i := n downto 1 do

s := s + i;

writeln('ket qua s :', s);

readln

end.

Hint 1: Loop

uses crt;
var n, res, i: longint;
begin
clrscr;
    readln(n);
    for i:=1 to n do inc(res, i);
    writeln(res);
readln;
end.

Hint 2: Math

uses crt;
var n: longint;
begin
clrscr;
    readln(n);
    writeln(n * (n + 1) div 2);
readln;
end.