Вращение вокруг одной из своих вершин

uses crt,graph;
procedure Vrash(a:real;var x,y:integer;x0,y0,d:integer);
begin
a:=a*pi/180;
x:=x0+round(d*cos(a));
y:=y0-round(d*sin(a));
end;
 
var gd,gm:integer;
    ch:char;
    x0,y0,d,ax,ay,bx,by:integer;
    aa,ab:real;
begin
clrscr;
repeat
writeln('Dlina storony (20-230) d=');
readln(d);
until d in [20..230];
writeln('Press Enter...');
readln;
gd:=0;
initgraph(gd,gm,'');
outtextXY(250,30,'Vyhod - Esc');
x0:=getmaxX div 2;y0:=getmaxY div 2;
aa:=60;ab:=120;
ax:=x0+round(d*cos(pi*aa/180));
ay:=y0-round(d*sin(pi*aa/180));
bx:=x0+round(d*cos(pi*ab/180));
by:=y0-round(d*sin(pi*ab/180));
Setcolor(12);
setlinestyle(0,0,3);
line(x0,y0,ax,ay);
line(ax,ay,bx,by);
line(x0,y0,bx,by);
repeat
delay(50);
setcolor(0);
setlinestyle(0,0,3);
line(x0,y0,ax,ay);
line(ax,ay,bx,by);
line(x0,y0,bx,by);
Vrash(aa,ax,ay,x0,y0,d);
Vrash(ab,bx,by,x0,y0,d);
Setcolor(12);
setlinestyle(0,0,3);
line(x0,y0,ax,ay);
line(ax,ay,bx,by);
line(x0,y0,bx,by);
aa:=aa+2;ab:=ab+2;
if keypressed then ch:=readkey;
until ch=#27;
end.