Ptolemy's theorem - maybe easy since only distances are involved
http://www.cut-the-knot.org/proofs/ptolemy.shtml
Is there any quick method of finding if 4 given pts are concyclic or not? (finding the eqn of the circle thru 3 pts and then checking the 4th pt in the eqn is really looooooooooong)
if ABCD are the point
<ACB=<ADC
Chord subtends equal angle
then they are concyclic!
Ptolemy's theorem - maybe easy since only distances are involved
http://www.cut-the-knot.org/proofs/ptolemy.shtml
yes this too..
it was my most favorite theorem in Geometry ptolmey's