Intersecting secants theorem

In Euclidean geometry, the intersecting secants theorem or just secant theorem describes the relation of line segments created by two intersecting secants and the associated circle. 

For two lines $AD$ and $BC$ that intersect each other at P and for which $A, B, C, D$ all lie on the same circle, the following equation holds: 

${\displaystyle |PA|\cdot |PD|=|PB|\cdot |PC|}$.

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