Triangle-Ception IV

Geometry Level 5

In the diagram above, O A D OAD is a quarter circle of radius 2. O B OB and O C OC trisect arc A D AD , and D E DE bisects segment O A OA . If the area of quadrilateral W X Y Z WXYZ can be expressed in the form

m n ( p q r ) \dfrac{m}{n}(p-q\sqrt{r})

for positive integers m , n , p , q m,n,p,q and r r with gcd ( m , n ) = 1 \gcd(m,n)=1 , gcd ( p , q ) = 1 \gcd(p,q)=1 , and r r is not divisible by the square of any prime. Find the value of

m + n + p + q + r . m+n+p+q+r.


The answer is 75.

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5 solutions

Mahdi Al-kawaz
Nov 24, 2015

Using basic facts we can obtain the following:

Z O D = W O Z = 3 0 , A D O = 4 5 , E D = 5 , sin E D O ^ = 1 5 \angle ZOD= \angle WOZ=30^\circ ,\angle ADO=45^\circ, \overline{ED}=\sqrt{5}, \sin \widehat{EDO}=\frac{1}{\sqrt{5}} and O W = O Z \overline{OW}=\overline{OZ} .

let E D O = α \angle EDO=\alpha \Rightarrow

sin O Y D ^ = sin ( 15 0 α ) = 2 + 3 2 5 \sin\widehat{OYD}=\sin(150^\circ-\alpha)=\frac{2+\sqrt{3}}{2\sqrt{5}}

sin O X D ^ = sin ( 12 0 α ) = 1 + 2 3 2 5 . \sin\widehat{OXD}=\sin(120^\circ-\alpha)=\frac{1+2\sqrt{3}}{2\sqrt{5}}.

Applying the law of sines on the triangles OWD, OXD and OYD we get O W = 4 1 + 3 \overline{OW}=\frac{4}{1+\sqrt{3}} ,

O X = 4 1 + 2 3 \overline{OX}=\frac{4}{1+2\sqrt{3}} and O Y = 4 2 + 3 \overline{OY}=\frac{4}{2+\sqrt{3}} respectively.

The area of WXYZ=area of OWZ-area of XOZ = 1 2 × sin 3 0 × O W × O Z 1 2 × sin 3 0 × O X × O Y \frac{1}{2} \times \sin 30^\circ \times \overline{OW} \times \overline{OZ}-\frac{1}{2} \times \sin 30^\circ \times \overline{OX} \times \overline{OY} = 2 11 ( 38 21 3 ) =\frac{2}{11} (38-21\sqrt{3})

Conclusion 1. The answer is 75 \boxed{75} .

Conclusion 2. Writing a solution using my phone..never again.

Great solution! Much more simple than mine!

And yeah... phones + latex never ends well...

Brandon Monsen - 5 years, 6 months ago
Akshay Yadav
Nov 24, 2015

I used coordinate geometry to solve the question, however my method is way too long and not appreciable enough to be shared. :|

Brandon Monsen
Nov 21, 2015

First, lets break down the diagram into sections in order to get the area of W X Y Z WXYZ . Let [ a ] [a] denote the area of a a .

We can tell by our diagram that

[ A O D ] = 2 = [ A W O ] + [ O Z D ] + [ O X Y ] + [ W X Y Z ] [\triangle AOD]=2=[\triangle AWO]+[\triangle OZD]+[\triangle OXY]+[WXYZ]

We can also place O O at the origin and let O A OA lie on the y a x i s y-axis and O D OD lie on the x a x i s x-axis .

We can represent every line that makes up W X Y Z WXYZ on the diagram as:

O B = y 1 = 3 x O C = y 2 = 1 3 x A D = y 3 = 2 x D E = y 4 = 1 1 2 x OB=y_{1}=\sqrt{3}x \\ OC=y_{2}=\frac{1}{\sqrt{3}}x \\ AD=y_{3}=2-x \\ DE=y_{4}=1-\frac{1}{2}x

We can find the areas of these triangles by dropping down perpendiculars and using A r e a = B a s e × H e i g h t Area=Base \times Height

To find [ A W O ] [\triangle AWO] we need the x-value of y 1 = y 3 y_{1}=y_{3} , so we get

3 x = 2 x x = 3 1 \sqrt{3}x=2-x \\ x=\sqrt{3}-1

And by A r e a = B a s e × H e i g h t Area = Base \times Height , we get

[ A W O ] = ( 3 1 ) ( 2 ) 2 = 3 1 [\triangle AWO]=\frac{(\sqrt{3}-1)(2)}{2}=\sqrt{3}-1

To Find [ O Z D ] [\triangle OZD] we need the y-value of y 2 = y 3 y_{2}=y_{3} , so we get

1 3 x = 2 x = y 2 = y 3 y 2 = y 3 = 3 1 \frac{1}{\sqrt{3}}x=2-x=y_{2}=y_{3} \\ y_{2}=y_{3}=\sqrt{3}-1

And so we can conclude that

[ A W O ] = [ O Z D ] = 3 1 [\triangle AWO]=[\triangle OZD]=\sqrt{3}-1

In order to find [ O X Y ] [\triangle OXY] , we can see that

[ E O D ] = 1 = [ O X Y ] + [ E X O ] + [ O Y D ] [\triangle EOD]=1=[\triangle OXY]+[\triangle EXO]+[\triangle OYD]

To find E X O \triangle EXO , we need the x-value of y 1 = y 4 y_{1}=y_{4} , so we get

3 x = 1 1 2 x x = 2 11 ( 2 3 1 ) \sqrt{3}x=1-\frac{1}{2}x \\ x=\frac{2}{11}(2\sqrt{3}-1)

So we can conclude that

[ E X O ] = ( 2 11 ( 2 3 1 ) ) ( 1 ) 2 = 1 11 ( 2 3 1 ) [\triangle EXO]=\frac{(\frac{2}{11}(2\sqrt{3}-1))(1)}{2}=\frac{1}{11}(2\sqrt{3}-1)

Also, to find [ O D Y ] [\triangle ODY] , we need the y-value of y 2 = y 4 y_{2}=y_{4} , so we get

1 3 x = 1 1 2 x = y 2 = y 4 y 2 = y 4 = 4 2 3 \frac{1}{\sqrt{3}}x=1-\frac{1}{2}x=y_{2}=y_{4} \\ y_{2}=y_{4}=4-2\sqrt{3}

So we can conclude that

[ E X O ] = 4 2 3 [\triangle EXO]=4-2\sqrt{3}

Now we have everything we need to finish the problem. We get that

[ W X Y Z ] = 2 [ A W O ] [ O Z D ] [ O X Y ] [ W X Y Z ] = 2 [ A W O ] [ O Z D ] ( 1 [ E X O ] [ O Y D ] ) [ W X Y Z ] = 2 ( 3 1 ) ( 3 1 ) ( 1 ( 4 2 3 ) ( 1 11 ( 2 3 1 ) ) ) [WXYZ]=2-[\triangle AWO]-[\triangle OZD]-[\triangle OXY] \\ [WXYZ]=2-[\triangle AWO]-[\triangle OZD]-(1-[\triangle EXO]-[\triangle OYD]) \\ [WXYZ]=2-(\sqrt{3}-1)-(\sqrt{3}-1)-(1-(4-2\sqrt{3})-(\frac{1}{11}(2\sqrt{3}-1)))

After some manipulation we get that

[ W X Y Z ] = 2 11 ( 38 21 3 ) [WXYZ]=\frac{2}{11}(38-21\sqrt{3})

This satisfies all of our constraints for m , n , p , q , r m,n,p,q,r , so

m + n + p + q + r = 2 + 11 + 38 + 21 + 3 = 75 m+n+p+q+r=2+11+38+21+3=\boxed{75}

Used the same method as Mahdi Al-kawaz . I was to give a little more detail. But I think his is sufficient.

However on second thought, coordinate geometry is also a nice method.
OD and OA, X and Y rectangular axis. Then, Y O B = 3 X , Y O C = 1 3 X , Y A D = X + 2 , Y E D 1 + X E D 2 = 1. Points of intersections give coordinates of W, X, Y, Z. So the area. \text{OD and OA, X and Y rectangular axis. Then,}\\ Y_{OB}=\sqrt 3 X,~~~~Y_{OC}=\dfrac 1 {\sqrt 3} X, ~~~~Y_{AD}= - X +2, ~~~ \dfrac{Y_{ED}} 1 + \dfrac{X_{ED}} 2 =1.\\ \text{Points of intersections give coordinates of W, X, Y, Z. So the area.}

Ahmad Saad
Dec 9, 2015

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