Inverse Kinematics Problem - Check Robotic Arm Reachability

#include<stdio.h>
#include<GRAPHICS.h>
#include<math.h>

double L1=110;
double L2=140;

double lx[3],ly[3];

double radian(int val)
{
  return (M_PI*val)/180;
}

void rotate(double x_c,double y_c,double a,double b,double *x_p,double *y_p,int degree)
{
   double rad=radian(degree);
   *x_p=((a-x_c)*cos(rad))-((b-y_c)*sin(rad))+x_c;
   *y_p=((a-x_c)*sin(rad))+((b-y_c)*cos(rad))+y_c;
}

int isreachable(int x,int y)
{
   int xc,yc,radius;


   if( ((x-lx[0])*(x-lx[0])+ (y-ly[0])*(y-ly[0])) > (L1+L2)*(L1+L2) )
   return 0;
    /* Case 2:                   */
    if(y>0)
    {
       xc=L1+lx[0];
       yc=ly[0];
       //radius=L2;
       if( ((x-xc)*(x-xc) + (y-yc)*(y-yc)) < L2*L2 )
       return 0;
    }
    /* Case 3:                    */
    if(y<=0)
    {
      xc=lx[0];
      yc=ly[0];
      radius=abs(L1)-abs(L2);
     if( ((x-lx[0])*(x-lx[0]) + (y-ly[0])*(y-ly[0])) < radius*radius )
     return 0;
    }
    /*Case 4:                      */
    if( y<=0 && x>=0)
    {
      xc=lx[0]-L1;
      yc=ly[0];
      radius=L2;
      if( ((x-xc)*(x-xc) + (y-yc)*(y-yc)) > radius*radius )
      return 0;

    }
    /*Case 5:                     */
    if( y<0 && x<0 )
    {
      xc=lx[0]-L1;
      yc=0;
      radius=L2;
      if( ((x-xc)*(x-xc) + (y-yc)*(y-yc)) > L2*L2 )
      return 0;
    }
    return 1;
}
double calculateAngle(double a,double b,double c)
{
      double val=(180/M_PI);
      double res=acos( ((a*a)+(b*b)-(c*c)) / (2*b*a)) * val;
      return res;
}
int main()
{
   double dx,dy,a,b;
   double a_rad,b_rad,tan_rad;
   int degree1,degree2;

  /* request auto detection */
   int gdriver = DETECT, gmode, errorcode;

  /* initialize graphics mode */
  initgraph(&gdriver, &gmode, "C:\\TurboC3\\BGI");

   lx[0]=getmaxx()/2;
   ly[0]=getmaxy()/2;

   printf("Origin-X: %lf Origin-Y: %lf\n",lx[0],ly[0]);

   lx[1]=lx[0]+L1;
   ly[1]=ly[0];

   lx[2]=lx[1];
   ly[2]=ly[1]-L2;

      setcolor(BLUE);
      line(lx[0],ly[0],lx[1],ly[1]);
      setcolor(RED);
      line(lx[1],ly[1],lx[2],ly[2]);

      printf("Enter dx :");
      scanf("%lf",&dx);
      printf("Enter dy :");
      scanf("%lf",&dy);

     // dx=lx[0]+dx;
    //  dy=ly[0]-dy;

      if(isreachable(lx[0]+dx,ly[0]-dy))
      {
 double dist;
       // setcolor(YELLOW);
       // line(lx[0],ly[0],dx,dy);
 dist=sqrt( (dx)*(dx) + (dy)*(dy) );

 b_rad=calculateAngle(L1,L2,dist);
 a_rad=calculateAngle(L1,dist,L2);

 printf("%lf ",a_rad);

 b_rad=(180-b_rad);

 tan_rad=((atan2(dy,dx))*(180/M_PI));

 printf("%lf\n",tan_rad);

 a_rad=(tan_rad-a_rad);

 printf("\nangle Alpha=%lf",a_rad);
 printf("\nangle Beta=%lf",b_rad);

 rotate(lx[0],ly[0],lx[1],ly[0],&a,&b,(0-a_rad));
 lx[1]=a;
 ly[1]=b;
 rotate(lx[0],ly[0],lx[0]+L1+L2,ly[0],&a,&b,(0-a_rad));
 lx[2]=a;
 ly[2]=b;

       //   printf("\n %lf %lf %lf %lf",lx[1],ly[1],lx[2],ly[2]);

 rotate(lx[1],ly[1],lx[2],ly[2],&a,&b,(0-b_rad));
 lx[2]=a;
 ly[2]=b;

      //    printf("\n%lf %lf",lx[2],ly[2]);
  clrscr();
  setcolor(BLUE);
  line(lx[0],ly[0],lx[1],ly[1]);
  setcolor(RED);
  line(lx[1],ly[1],lx[2],ly[2]);
  //setcolor(YELLOW);
  //line(lx[0],ly[0],lx[2],ly[2]);
      }
      else
      {
  printf("Destination is unreachable");
      }

   getch();

   return 0;
}

Input :

Output :