Saltire Software

Ptolemy's Theorem

Ptolemy's Theorem gives a relationship between the sides of a cyclic quadrilateral and its diagonals.

NOTE: A cyclic quadrilateral is one that can be inscribed inside a circle. Given any constructible quadrilateral, it can always be changed so that it is cyclic.

See if you can spot the relationship Ptolemy discovered ages ago.

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AD
AB
BC
CD
AC
BD
AC * BD
AB*CD + AD*BC

App generated by Geometry Expressions