MHB Inscribed Quadrilateral Perpendicular Bisectors Convergence Theorem

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In the discussion on the Inscribed Quadrilateral Perpendicular Bisectors Convergence Theorem, participants explore the relationship between the lengths of segments PQ and QR and the intersection of angle bisectors of angles ABC and ADC on line AC. The problem states that PQ equals QR if and only if the specified angle bisectors converge at that point. The thread emphasizes the geometric properties of inscribed quadrilaterals and their perpendiculars. Participants are encouraged to engage with the Problem of the Week and refer to the suggested solution for guidance. The lack of responses to the previous week's problem highlights the need for more interaction in the forum.
anemone
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Here is this week's POTW:

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Let $ABCD$ be an inscribed quadrilateral. Let $P$, $Q$ and $R$ be the feet of the perpendiculars from $D$ to the lines $BC$, $CA$ and $AB$ respectively. Show that $PQ = QR$ if and only if the bisectors of $\angle ABC$ and $\angle ADC$ meet on $AC.$

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Remember to read the https://mathhelpboards.com/showthread.php?772-Problem-of-the-Week-%28POTW%29-Procedure-and-Guidelines to find out how to https://mathhelpboards.com/forms.php?do=form&fid=2!
 
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No one answered last week's POTW. (Sadface) You can refer to the suggested solution as shown below.

View attachment 9265

Denote in the triangle $ABC$ the angles at $A$ and $C$ by $\alpha$ and $\gamma$ respectively. $P$ and $Q$ are incident to the Thales circle of $CD$ and $\sin \angle PCQ=\sin \gamma$ or $PQ=CD\sin \gamma$.

Similarly, $QR=AD\sin \alpha$. By condition

$PQ=CD \sin \gamma=AD\sin \alpha=QR$

Rearranging and applying the sine rule for the triangle $ABC$ yields

$\dfrac{CD}{AD}=\dfrac{\sin \alpha}{\sin \gamma}=\dfrac{CB}{AB}$

By the angle bisector theorem, this holds if and only if the bisectors of $\angle ABC$ and $\angle ADC$ meet the segment $AC$ at the very same point.
 

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