Ptolemy's Theorem
   

   

 Math Help -> Geometry and Trigonometry -> Polygons and Triangles -> Cyclic quadrilateral -> Ptolemy's theorem 

Ptolemy's Theorem

This theorem was proved by Giovanni Ceva (1648-1734).

Ptolemy's Theorem Ptolemy's theorem states that given a cyclic quadrilateral (i.e. one that can be inscribed in a circle) the product of the diagonals equals the sum of the products of opposite sides.

On the diagonal BD locate a point M such that angles BCA and MCD are equal.  Since angles BAC and MDC subtend the same arc, they are equal.  (why?)  Therefore, triangles ABC and DMC are similar.

Thus we get CD/MD = AC/AB, or AB·CD = AC·MD.

Since angles BCA and MCD are equal, then angle BCM=BCA+ACM equals angle ACD=ACM+MCD.  So triangles BCM and ACD are similar which leads to

BC/BM = AC/AD, or BC·AD = AC·BM.

Summing up the two identities we obtain

AB·CD + BC·AD = AC·MD + AC·BM = AC·BD

Related pages in this website:

Summary of geometrical theorems

Inscribed Angles -- a proof that all angles inscribed in a circle subtend an arc that is twice the arc subtended by the same central angle.

Ptolemy's Inequality -- a generalization of Ptolemy's Theorem for convex quadrilaterals that aren't necessarily cyclic

Triangles -- a home page for anything about triangles and other polygons.

References

http://cut-the-knot.com/proofs/ptolemy.html gives this proof along with the interesting fact about the three chords formed by the vertices of an equilateral triangle and any fourth point on the circle: the sum of the shorter two equals the longer.

 

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