Ptolemy's theorem for a cyclic quadrilateral $ABCD$ with diagonals $AC$, $BD$ states —

Question

Accepted Answer

$AC \cdot BD = AB \cdot CD + BC \cdot AD$

Answer

$AC \cdot BD = AB \cdot BC + CD \cdot AD$

Answer

$AC + BD = AB + BC + CD + AD$

Answer

$AC \cdot BD = AB \cdot AD - BC \cdot CD$

Ptolemy's theorem for a cyclic quadrilateral ABCDABCDABCD with diagonals ACACAC, BDBDBD states —