# vector , geometry

• December 31st 2008, 12:30 PM
Tweety
vector , geometry
In triangle PQR, the line ST is drawn parallel to QR so that PS= 3SQ. prove that PT = 3TR
• January 1st 2009, 10:57 AM
earboth
Quote:

Originally Posted by Tweety
In triangle PQR, the line ST is drawn parallel to QR so that PS= 3SQ. prove that PT = 3TR

1. Let $PT = k \cdot TR$

2. You are dealing with similar triangles. So use proportion:

$\dfrac{PS}{PS + SQ} = \dfrac{PT}{PT + TR}$

3. Substitute

$PS= 3 \cdot SQ$ ...... and ...... $PT = k \cdot TR$

$\dfrac{3 \cdot SQ}{3 \cdot SQ + SQ} = \dfrac{k \cdot TR}{k \cdot TR + TR}$

$\dfrac{3 \cdot SQ}{4 \cdot SQ } = \dfrac{k \cdot TR}{(k+1) \cdot TR }$

$\dfrac{3}{4 } = \dfrac{k}{(k+1) } ~\implies~3(k+1) = 4k ~\implies~\boxed{3=k}$
• January 1st 2009, 12:11 PM
Tweety
thanks so much for that, I would have never figured out how to prove it!