C graphics program to rotate an object about the origin

/* This program will  rotate an object ( triangle) about the origin */

#include<graphics.h>
#include<conio.h>
#include<stdio.h>
#include<math.h>
void main()
{
int gd=DETECT,gm;
int i,xmid,ymid,x1,y1,x2,y2,x3,y3,x,y,dy,dx,p,gap=50,temp;
int x1dash,x2dash,x3dash,y1dash,y2dash,y3dash;
float m;
double theta;
char str[5];
clrscr();
initgraph(&gd,&gm,"..\\bgi");

printf("Enter first co-ords of the triangle\n");
scanf("%d %d",&x1,&y1);
printf("Enter second co-ords of the triangle\n");
scanf("%d  %d",&x2,&y2);
printf("Enter third co-ords of the triangle\n");
scanf("%d  %d",&x3,&y3);

xmid= getmaxx()/2;
ymid= getmaxy()/2;
line(5,ymid,getmaxx()-5,ymid);
line(xmid+3,5,xmid+3,getmaxy()-5);

for( i= xmid+gap;i<getmaxx()-5;i=i+gap)
{
outtextxy(i,ymid-3,"|");
itoa(i-xmid,str,10);
outtextxy(i,ymid+3,str);
}
for( i= ymid-gap;i>5;i=i-gap)
{
outtextxy(xmid,i,"-");
itoa(ymid-i,str,10);
outtextxy(xmid+5,i,str);

}
for( i= xmid-gap;i>5;i=i-gap)
{
outtextxy(i,ymid-3,"|");
itoa(-(xmid-i),str,10);
outtextxy(i-6,ymid+3,str);

}
for( i= ymid+gap;i<getmaxy()-5;i=i+gap)
{
outtextxy(xmid,i,"-");
itoa(-(i-ymid),str,10);
outtextxy(xmid+8,i,str);
}
line(x1+xmid,ymid-y1,x2+xmid,ymid-y2);
line(x2+xmid,ymid-y2,x3+xmid,ymid-y3);
line(x3+xmid,ymid-y3,x1+xmid,ymid-y1);

printf("Enter the degree to rotate");
scanf("%lf",&theta);
theta= ((float) theta *3.14f )/(float)180;
x1dash=x1*cos(theta)-y1*sin(theta);
x2dash=x2*cos(theta)-y2*sin(theta);
x3dash=x3*cos(theta)-y3*sin(theta);
y1dash=x1*sin(theta)+y1*cos(theta);
y2dash=x2*sin(theta)+y2*cos(theta);
y3dash=x3*sin(theta)+y3*cos(theta);

line(x1dash+xmid,ymid-y1dash,x2dash+xmid,ymid-y2dash);
line(x2dash+xmid,ymid-y2dash,x3dash+xmid,ymid-y3dash);
line(x3dash+xmid,ymid-y3dash,x1dash+xmid,ymid-y1dash);

getch();
closegraph();
}


Sample Input :

  x1 ,y1   :  50   50
  x2, y2   : 100 100
  x3 ,y3   :150   50