MHB How Many Integer Values Can QR Take in Triangle PQR with Equal Area Divisions?

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In triangle PQR, with PQ equal to 7, the angle bisector QPR intersects QR at point A. Points B and C on sides PR and QR, respectively, create parallel lines PA and BC that divide triangle PQR into three equal-area sections. The discussion focuses on determining the integer values that QR can take under these conditions. The solutions provided by members laura123 and johng contribute to solving this geometric problem. The final goal is to find the total number of possible integer values for QR.
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Let $PQR$ be a triangle such that $PQ=7$ and let the angle bisector $QPR$ intersect line $QR$ at $A$. If there exist points $B$ and $C$ on sides $PR$ and $QR$ respectively, such that lines $PA$ and $BC$ are parallel and divide triangle $PQR$ into three parts of equal area, determine the number of possible integer values for $QR$.

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Hello MHB Community,

anemone is a bit under the weather this week, so she has asked me to fill in for her. Please join me in wishing for her a speedy recovery. (Yes)

Congratulations to the following members for their correct solutions:

  1. laura123
  2. johng

Solution from laura123:

The points $B$ and $C$ (such that lines $PA$ and $BC$ are parallel and divide triangle $PQR$ into three parts of equal area) there exist if the area of the triangle $PRA$ is equal to twice the area of the triangle $PAQ$.
Let $A_1$ be the area of the triangle $PAQ$ and $A_2$ the area of the triangle $PRA$.
$A_2=2A_1$ if $PR=2PQ$ in fact:
$A_1=\dfrac{1}{2}PQ\cdot PA\cdot \sin(\angle QPA)$;
$A_2=\dfrac{1}{2}PR\cdot PA\cdot \sin(\angle APR)$.
Since $\angle QPA=\angle APR$ ($PA$ is the bisector of $\angle QPR$) and $A_2=2A_1$ it follows $PR=2PQ$.
Since $PQ=7$, we have $PR=14$.
10dyvs2.png

Therefore, the vertex $R$ must belong to a circle with centre $P$ and radius $14$, as shown in the following figure:
2hmhrly.png

then: $7<QR<21$. Therefore, the possible integer values for $QR$ are:
8,9,10,11,12,13,14,15,16,17,18,19,20.
There are 13 possible integer values for $QR$.

Solution from johng:

2rms3mx.png


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