## Quadrilateral Balancing Theorem: Another 'Van Aubel-like' theorem

Quadrilateral Balancing theorem: Given four points A, B, C, D, and four directly similar quadrilaterals AP1P2B, CQ1Q2B, CR1R2D, AS1S2D with respective centroids P, Q, R, S. Let K, L, M and N be the midpoints of the segments P1Q2, Q1R2, R1S2 and S1P2 respectively (or of P1S2, S1R2, R1Q2 and Q1P2), and let V, W, X be the centroids of the quadrilaterals ABCD, PQRS, KLMN respectively. Then:
(i) PQRS is a parallelogram
(ii) KLMN is a parallelogram, and
(iii) W is the midpoint of the segment VX.

#### .sketch_canvas { border: medium solid lightgray; display: inline-block; } Quadrilateral Balancing Theorem: Another 'Van Aubel-like' theorem (2007)

For a proof of the above, read my 2007/2008 article in the Mathematical Gazette: A Question of Balance: An Application of Centroids.

Similar Triangles on Sides of Quadrilateral Theorem: Given a quadrilateral ABCD with similar triangles PBA, QBC, RDC and SDA constructed on the sides so that say the first and third are exterior to the quadrilateral, while the second and the fourth are interior to the quadrilateral, then quadrilateral PQRS is a parallelogram.

Important: To view & manipulate the dynamic version of this result, navigate to it using the appropriate button in the ABOVE dynamic sketch; the picture below is static.

#### Directly Similar Triangles on Sides of Quadrilateral Theorem (1994)

A proof of the above result appears on pp. 145-148 of the (1994/2009) edition of my book Some Adventures in Euclidean Geometry as well as in my paper "The Role of Proof in Investigative, Computer-based Geometry: Some personal reflections", a chapter in the 1997 MAA book, Geometry Turned On!, edited by Doris Schattschneider & Jim King.

Lemma: A Trio of Parallelograms: Given two parallelograms ABCD and IJKL, the midpoints E, F, G, H of the segments AI, BJ, CK, DL form another parallelogram. More-over, the centre Y of EFGH is the midpoint of XZ where X and Z are respective centres of ABCD and IJKL. (The diagram shows a case with the parallelograms both labelled anticlockwise, but both lemma and proof work equally well in all cases.)

Important: To view & manipulate the dynamic version of this result, navigate to it using the appropriate button in the ABOVE dynamic sketch; the picture below is static.

#### Lemma: A Trio of Parallelograms (2007)

The lemma above, together with the Directly Similar Triangles Theorem, was used to prove the Quadrilateral Balancing Theorem in my 2007/2008 article in the Mathematical Gazette: A Question of Balance: An Application of Centroids.

Van Aubel's theorem: Here is a link to Van Aubel's theorem and some Generalizations.

An Associated Result of Van Aubel & its Generalization (2022): For an interesting associated result of Van Aubel's theorem also involving similar quadrilaterals on the sides, go to An associated result of the Van Aubel configuration and some generalizations.

Created 2013 by Michael de Villiers. Most recent updates, 16 March 2020; 19 February 2022.