Вращение прямоугольника вокруг точки

program three;
uses crt,GraphABC;
var
a,b: integer;
d,m : smallint;
x1,x2,x3,x4,y1,y2,y3,y4: integer;
alpha,beta,koef : real;
x0,y0 : integer;
ch : char;
begin
clrscr;
alpha := 0;
koef := 1;
x0 := 300;  
y0 := 300;
a := 100;   
b := 200;   
beta := arctan(b/a);
repeat
line(x1,y1,x2,y2);
line(x2,y2,x3,y3);
line(x3,y3,x4,y4);
line(x4,y4,x1,y1);
 
x1 := x0;
y1 := y0;
x2 := x0+round(a*cos(alpha));
y2 := y0+round(a*sin(alpha));
x3 := x0+round(sqrt(sqr(a)+sqr(b))*cos(alpha-beta));
y3 := y0+round(sqrt(sqr(a)+sqr(b))*sin(alpha-beta));
x4 := x0+round(b*cos(alpha-pi/2));
y4 := y0+round(b*sin(alpha-pi/2));
 
alpha := alpha + koef*0.1;
if alpha>=2*pi then alpha := 0;
delay(70);
cleardevice;
 
if keypressed then
 begin
  ch := readkey;
  if ch = '1' then koef := 1
  else if ch='2' then koef := -1
  else if ch=#27 then break;
 end;
until ch=#27;
 
 
end.

program three;
 
uses
  crt, GraphABC;
 
var
  a, b: integer;
  d, m: smallint;
  x1, x2, x3, x4, y1, y2, y3, y4: integer;
  alpha, beta, koef: real;
  x0, y0: integer;
  ch: char;
 
begin
  clrscr;
  alpha := 0;
  koef := 1;
  x0 := 300;  
  y0 := 300;
  a := 100;   
  b := 200;   
  beta := arctan(b / a);
  repeat
    lockdrawing;
    clearwindow;
    line(x1, y1, x2, y2);
    line(x2, y2, x3, y3);
    line(x3, y3, x4, y4);
    line(x4, y4, x1, y1);
    
    x1 := x0;
    y1 := y0;
    x2 := x0 + round(a * cos(alpha));
    y2 := y0 + round(a * sin(alpha));
    x3 := x0 + round(sqrt(sqr(a) + sqr(b)) * cos(alpha - beta));
    y3 := y0 + round(sqrt(sqr(a) + sqr(b)) * sin(alpha - beta));
    x4 := x0 + round(b * cos(alpha - pi / 2));
    y4 := y0 + round(b * sin(alpha - pi / 2));
    
    alpha := alpha + koef * 0.1;
    if alpha >= 2 * pi then alpha := 0;
    delay(70);
    redraw;
    
    if keypressed then
    begin
      ch := readkey;
      if ch = '1' then koef := 1
      else if ch = '2' then koef := -1
      else if ch = #27 then break;
    end;
  until ch = #27;
  
  
end.