Proof of a property of elipse tangents. Angles and segments with the same colour are equal. Orange angle is equal to red angles.
<a href="//commons.wikimedia.org/w/index.php?title=User:MatFizka&amp;action=edit&amp;redlink=1" class="new" title="User:MatFizka (page does not exist)">MatFizka</a>
CC BY 1.0
<span class="int-own-work" lang="en">Own work</span>