The equation in Ptolemy's theorem is never true with non-cyclic quadrilaterals. Ptolemy's inequality is an extension of this fact, and it is a more general form of Ptolemy's theorem. It states that, given a quadrilateral ABCD, then
{displaystyle {overline {AB}}cdot {overline {CD}}+{overline {BC}}cdot {overline {DA}}geq {overline {AC}}cdot {overline {BD}}} overline{AB}cdot overline{CD}+overline{BC}cdot overline{DA} ge overline{AC}cdot overline{BD}
where equality holds if and only if the quadrilateral is cyclic. This special case is equivalent to Ptolemy's theorem.
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