For a quadrilateral which is not cyclic, Ptolemy's theorem becomes an inequality:
The Ptolemy inequality is still valid when is a triangular pyramid (Boomstra 1956-1957).
For a quadrilateral which is not cyclic, Ptolemy's theorem becomes an inequality:
The Ptolemy inequality is still valid when is a triangular pyramid (Boomstra 1956-1957).
Weisstein, Eric W. "Ptolemy Inequality." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/PtolemyInequality.html