Bài 1 : 

uses crt;

var n, i, s : integer;

begin

clrscr;

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

s := 0;

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

writeln('tong s = ', s);

readln

end.

Bài 2 :

uses crt;

var a, b, bc : integer;

begin

clrscr;

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

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

bc := a * b;

while a <> b do

if a > b then a := a - b else b := n - a;

writeln('boi chung nho nhat la :', bc div a);

readln

end.

Bài 3 : 

uses crt;

var a, b, c : real;

begin

clrscr;

write('nhap he so a, b, c :'); readln(a, b, c);

dt := b * b - 4 * a * c;

if a <> 0 then

begin

if dt >0 then

begin

writeln('nghiem x1 =', ((- b + sqrt(dt))/(2 * a)):2:2);

writeln('nghiem x2 =', ((- b - sqrt(dt))/(2 * a)):2:2);

end

else if dt = 0 then writeln('x = ', (-b / (2 * a)):2:2);

else writeln(' Phương trinh vo nghiem');

end;

readln;

end.