Apply Morley's Theorem?

Geometry Level 5

Let A B C ABC be a 6-8-10 triangle. Call the points of intersection of adjacent angle trisectors of different angles be X X , Y Y and Z Z . Find (to 3 decimal places) the area of X Y Z XYZ .


The answer is 0.726.

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3 solutions

Maria Kozlowska
Jun 6, 2016

Let r r be circumradius of A B C \triangle ABC ,

α = A / 3 , β = B / 3 , γ = C / 3 \alpha=\angle A/3, \beta=\angle B/3, \gamma=\angle C/3

Side of a Morley triangle is:

s = 8 r sin ( α ) sin ( β ) sin ( γ ) = s=8 r* \sin(\alpha) \sin(\beta)\sin(\gamma)= 8 5 sin ( 30 ) sin ( arctan ( 3 / 4 ) / 3 ) sin ( arctan ( 4 / 3 ) / 3 ) 1.295 8 * 5* \sin(30) \sin( \arctan(3 / 4)/ 3) \sin( \arctan(4 / 3)/ 3)\approx 1.295

A r e a = 3 4 s 2 0.726 Area=\frac{\sqrt{3}}{4} s^2\approx \boxed{0.726}

Ken Hodson
Jun 6, 2016

I just used brute force after plotting the triangle on a Cartesian Plane. I just banged out Point-Slope equations to find X,Y and Z (with paper and pencil (and calculator for trig ratios and long division) - and it took about half an hour using basic algebra techniques). I was humbled to see that XYZ is an equilateral triangle. There's got to be a simpler way.

link text
L e t α = X / 3 , β = Y / 3 , γ = Z / 3. T h e c i r c u m r a d i u s R = x y z 4 a r e a X Y Z = 6 8 10 4 1 2 6 8 = 5. I n t h e a b o v e l i n k t h e Δ X Y Z i s n a m e d a s Δ P Q R , a n d o r i g i n a l t r i a n g l e i s i n s c r i b e d i n a u n i t c i r c l e . T h e f o r m u l a g i v e n i s P Q = 8 S i n ( α ) S i n ( β ) S i n ( γ ) . I n o u r c a s e t h i s w o u l d t r a n s l a t e i n t o , X Y = 8 R S i n ( α ) S i n ( β ) S i n ( γ ) B u t M o r l e y s T h e o r e m s t a t e s t h a t t h i s i s a n e q u i l a t e r a l Δ . t h e a r e a o f X Y Z = 3 4 { R S i n ( α ) S i n ( β ) S i n ( γ ) } 2 a r e a o f X Y Z = 3 4 { 5 S i n ( S i n 1 . 8 3 ) S i n ( S i n 1 . 6 3 ) S i n ( S i n 1 1 3 ) } 2 a r e a o f X Y Z = 0.7262 Let\ \alpha=\angle X/3, \beta=\angle Y/3, \gamma=\angle Z/3.\\ The \ circumradius \ R=\dfrac{xyz}{4*area\ XYZ}=\dfrac{6*8*10}{4*\frac 1 2 *6*8}=5.\\ In\ the\ above\ link\ the\ \Delta\ XYZ\ is\ named\ as\ \Delta\ PQR,\ and\ original\ triangle\ is inscribed \ in\ a\ unit\ circle.\\ The\ formula\ given\ is\ PQ\ =\ 8*Sin(\alpha)*Sin(\beta)*Sin(\gamma).\\ In\ our\ case\ this\ would\ translate\ into,\\ XY=8 R* Sin(\alpha) Sin(\beta)Sin(\gamma)\\ But\ Morley's\ Theorem\ states\ that\ this\ is\ an\ equilateral\ \Delta.\\ \therefore\ the\ area\ of\ XYZ=\frac{\sqrt3} 4 *\{R*Sin(\alpha)Sin(\beta)Sin(\gamma)\}^2\\ \implies\ area \ of\ XYZ\ =\ \frac{\sqrt3} 4 *\left\{5*Sin\left(\dfrac{Sin^{-1} .8} 3\right)*Sin\left(\dfrac{Sin^{-1} .6} 3\right)*Sin\left(\dfrac{Sin^{-1} 1} 3\right) \right \}^2\\ area \ of\ XYZ\ =\large \ \ \ \ \color{#D61F06}{0.7262}

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