Dùng for:

program ct;
uses crt;
var n,i,SUM: int64;
begin
    clrscr;
    writeln('Nhap N: '); readln(n);
    SUM := 0;
    for i:= 1 to N do
        SUM:=SUM+i;
    writeln('Tong 1+..+',n,' = ',SUM);
    readln;
end.

------------------------

Dùng repeat - until:

program ct1;
uses crt;
var n,i,SUM: int64;
begin
    clrscr;
    writeln('Nhap N: '); readln(n);
    SUM := 0; i:=0;
    repeat
        i:=i+1;
        SUM:=SUM+i;
    until i = n;
    writeln('Tong 1+..+',n,' = ',SUM);
    readln;
end.

____________________________

#Duahauvn

Cách 1:

uses crt;

var s,n,i:integer;

begin

clrscr;

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

s:=0;

reapeat

s:=s+i;

i:=i+1;

until (i > n);

write('s bang: ',s);

readln;

end.

Cách 2:

uses crt;

var s,n,i:integer;

begin

clrscr;

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

s:=0;

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

write('s bang: ',s);

readln;

end.