title: How to calculate diametrically opposite point inside a circle?
body: <p>x1,y1 is a point inside a circle (not in the circumference of the circle). How can I calculate the diametrically opposite point?</p>
  
  <pre>
         |
         |   x1,y1
         |
  -------|--------
         |
  x2,y2  |
         |
  </pre>