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**Quasi-circumcentre**: Given a quadrilateral *ABCD* as shown below. Let *K*, *L*, *M*, and *N* be the respective circumcentres of triangles *ABD*, *ADC*, *BCD* and *ABC*, then the intersection *O* of *KM* and *LN* is equidistant from opposite vertices *A* and *C*, as well as equidistant from opposite vertices *B* and *D*. The point *O* is called the *quasi-circumcentre* of *ABCD*.

Quasi-circumcentre of quadrilateral

**Renate's Theorem**: The above result about the quasi-circumcentre was experimentally discovered and proved from a problem posed in [1] to find the “best” place to build a water reservoir for four villages of more or less equal size, if the four villages are not concyclic. It followed from the classroom discussion of a proposed solution by an undergraduate student, Renate Lebleu Davis, at Kennesaw State University during 2006.

This result was used in the Kennesaw State Mathematics Competition for High School students in 2007, as well as in the World InterCity Mathematics Competition for Junior High School students in 2009.

**Reference**: [1] M. de Villiers, *Rethinking Proof with Sketchpad*, Emeryville: Key Curriculum Press, 1999/2003.

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**Quasi-incentre**: Given a quadrilateral *ABCD* as shown below. Construct the angle bisectors for each of the four angles. Label *E* the intersection of the angle bisectors of angles *A* and *B*, label *F* the intersection of the angle bisectors of angles *B* and *C*, label *G* the intersection of the angle bisectors of angles *C* and *D*, and label *H* the intersection of the angle bisectors of angles *D* and *A*. Then *I* the intersection of *EG* and *FH* is equidistant from opposite sides *AD* and *BC*, as well as equidistant from opposite sides *AB* and *CD*. The point *I* is called the *quasi-incentre* of *ABCD*

Quasi-incentre of quadrilateral

**Comment**: Note that both above results remain valid if *ABCD* is concave or crossed - check by dragging!

Both results above can easily be proved using the respective properties of perpendicular bisectors and angles bisectors, e.g. see proofs.

**Further generalization & application**: The quasi-circumcentre can be extended to a quadrilateral and a hexagon where it is collinear with the so-called *quasi-orthocentre* of a quadrilateral and *centre of gravity*, lying on the so-called quasi-Euler line of a quadrilateral and a hexagon.

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By Michael de Villiers. Created, 23 November 2014.