A Visual Proof of Ptolemy's Theorem

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A significant result in classical geometry is Ptolemy's theorem: in a cyclic quadrilateral, the sum of the products of the two pairs of opposite sides is equal to the product of the diagonals. This Demonstration presents a visual proof of the theorem, based on [1].

Hover over

α

,

β

,

γ

,

δ

until you see a plus sign; then you can drag that vertex in its quadrant.