Ptolemy's Inequality
Ptolemy's Inequality states that in for four points in the plane,
,
with equality if and only if is a cyclic quadrilateral with diagonals
and
.
Proof
We construct a point such that the triangles
are similar and have the same orientation. In particular, this means that
.
But since this is a spiral similarity, we also know that the triangles are also similar, which implies that
.
Now, by the triangle inequality, we have . Multiplying both sides of the inequality by
and using
and
gives us
,
which is the desired inequality. Equality holds iff. ,
, and
are collinear. But since the angles
and
are congruent, this would imply that the angles
and
are congruent, i.e., that
is a cyclic quadrilateral.