Concurrent Cevians

Geometry Level 3

In Q R S \triangle QRS , P P is the point of concurrency for cevians A Q AQ , B R BR , and C S CS so that the areas of A P R \triangle APR , B P S \triangle BPS , and C P Q \triangle CPQ are 168 168 , 90 90 , and 14 14 , respectively.

Find the area of Q R S \triangle QRS .


The answer is 595.

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1 solution

Mark Hennings
Feb 6, 2019

If the areas of A P S APS , B P Q BPQ and C P R CPR are x , y , z x,y,z respectively, the Ceva's Theorem tells us that x 168 × z 14 × y 90 = 1 \frac{x}{168} \times \frac{z}{14} \times \frac{y}{90} \; = \; 1 so that x y z = 211680 xyz= 211680 . Comparing the areas of A P S APS , A Q S AQS , A P R APR and A Q R AQR tells us that x 168 = x + y + 90 z + 182 \frac{x}{168} \; = \; \frac{x+y+90}{z+182} and, similarly z 14 = x + z + 168 y + 104 y 90 = y + z + 14 x + 258 \frac{z}{14} \; = \; \frac{x+z+168}{y+104} \hspace{2cm} \frac{y}{90} \; = \; \frac{y+z+14}{x+258} Solving these various identities gives only one positive solution for x , y , z x,y,z , namely x = 252 x=252 , y = 15 y=15 , z = 56 z=56 . This makes the area of the triangle Q R S QRS equal to 595 \boxed{595} .

Great solution!

David Vreken - 2 years, 4 months ago

Oh no ! This is exactly what I did but got the answer 585

Miss Physicist - 2 years, 4 months ago

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