Let Δ ABC be an equilateral triangle. Let P be a point on minor arc AB of its circumcircle . Which of the following holds :
Draw PA,PB,PC. By Ptolemy's Theorem applied to quadrilateral APBC,
we know that PC×AB=PA×PB+PB×AC.
Since,AB=BC=CA=s, we divide both sides of the last equation by to get the result:PC=PA+PB.