Bằng lệnh for...do

uses crt;

var s,i,n:integer;

begin

clrscr;

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

s:=0;

for i:=1 to n do 

  s:=s+i;

writeln(s);

readln;

end.

Bằng lệnh while...do

uses crt;

var s,i,n:integer;

begin

clrscr;

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

s:=0;

i:=1;

while i<=n do 

  begin

s:=s+i;

inc(i);

end;

writeln(s);

readln;

end.

- Sử dụng for .. do :

uses crt;

var i, n, S : integer;

begin

clrscr;

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

S := 0;

for i := 1 to n do S := S + i;

write('S = 1 + 2 + ... + ', n ,' = ', S);

readln

end.

- Sử dụng while .. do :

uses crt;

var i, n, S : integer;

begin

clrscr;

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

S := 0; i := 1;

while i <= n do 

begin S := S + i;

i := i + 1; end;

write('S = 1 + 2 + ... + ', n ,' = ', S);

readln

end.