What's the Area of the Region?

Geometry Level pending

In the diagram above the two regions are labeled R R and B B .

If the area of the pink region A P = 1 2 A_{P} = \dfrac{1}{2} and A R = α β β ( ω β arctan ( β ) α arctan ( β ) β ) A_{R} = \dfrac{\sqrt{\alpha}}{\beta^{\beta}} (\dfrac{\omega - \beta\arctan(\beta)}{\sqrt{\alpha}\arctan(\beta) - \beta}) , where α , β \alpha, \beta ω \omega are coprime positive integers, find α + β + ω \alpha + \beta + \omega .


The answer is 10.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

2 solutions

Rocco Dalto
Feb 5, 2021

A B E F M E 2 1 = a F M F M = a 2 A F M E = a 2 4 \triangle{ABE} \sim \triangle{FME} \implies \dfrac{2}{1} = \dfrac{a}{\overline{FM}} \implies \overline{FM} = \dfrac{a}{2} \implies A_{\triangle{FME}} = \dfrac{a^2}{4}

and tan ( θ ) = 1 2 θ = arctan ( 1 2 ) A s e c t o r ( F P E ) = 1 2 arctan ( 1 2 ) a 2 \tan(\theta) = \dfrac{1}{2} \implies \theta = \arctan(\dfrac{1}{2}) \implies A_{sector(FPE)} = \dfrac{1}{2}\arctan(\dfrac{1}{2})a^2 \implies

A B = A F M E A s e c t o r ( F P E ) = ( 1 4 1 2 arctan ( 1 2 ) ) a 2 A_{B} = A_{\triangle{FME}} - A_{sector(FPE)} = (\dfrac{1}{4} - \dfrac{1}{2}\arctan(\dfrac{1}{2}))a^2 \implies

A R = a 2 π 4 a 2 A B = ( 3 4 + 1 2 arctan ( 1 2 ) π 4 ) a 2 A_{R} = a^2 - \dfrac{\pi}{4}a^2 - A_{B} = (\dfrac{3}{4} + \dfrac{1}{2}\arctan(\dfrac{1}{2}) - \dfrac{\pi}{4})a^2

To Find a 2 a^2 using A P = 1 2 A_{P} = \dfrac{1}{2} :

B E = 5 \overline{BE} = \sqrt{5} and tan ( λ ) = tan ( π 2 θ ) = 1 tan ( θ ) = 2 \tan(\lambda) = \tan(\dfrac{\pi}{2} - \theta) = \dfrac{1}{\tan(\theta)} = 2 \implies

A s e c t o r ( P D E ) = 1 2 arctan ( 2 ) a 2 A_{sector(PDE)} = \dfrac{1}{2}\arctan(2)a^2 and h = a sin ( λ ) = a cos ( θ ) = 2 5 a h = a\sin(\lambda) = a\cos(\theta) = \dfrac{2}{\sqrt{5}}a \implies

A P D E = 1 2 ( a ) ( 2 a 5 ) = a 2 5 A_{\triangle{PDE}} = \dfrac{1}{2}(a)(\dfrac{2a}{\sqrt{5}}) = \dfrac{a^2}{\sqrt{5}} \implies A P = 1 2 = A s e c t o r ( P D E ) A P D E = A_{P} = \dfrac{1}{2} = A_{sector(PDE)} - A_{\triangle{PDE}} =

( 5 arctan ( 2 ) 2 2 5 ) a 2 a 2 = 5 5 arctan ( 2 ) 2 (\dfrac{\sqrt{5}\arctan(2) - 2}{2\sqrt{5}})a^2 \implies a^2 = \dfrac{\sqrt{5}}{\sqrt{5}\arctan(2) - 2}

Using the fact that θ = arctan ( 1 2 ) \theta = \arctan(\dfrac{1}{2}) and λ = π 2 θ = arctan ( 2 ) \lambda = \dfrac{\pi}{2} - \theta = \arctan(2) \implies

θ = π 2 arctan ( 2 ) \theta = \dfrac{\pi}{2} - \arctan(2) \implies A R = 5 4 ( 3 2 arctan ( 2 ) 5 arctan ( 2 ) 2 ) = A_{R} = \dfrac{\sqrt{5}}{4}(\dfrac{3 - 2\arctan(2)}{\sqrt{5}\arctan(2) - 2}) =

α β β ( ω β arctan ( β ) α arctan ( β ) β ) \dfrac{\sqrt{\alpha}}{\beta^{\beta}} (\dfrac{\omega - \beta\arctan(\beta)}{\sqrt{\alpha}\arctan(\beta) - \beta}) \implies α + β + ω = 10 \alpha + \beta + \omega = \boxed{10} .

David Vreken
Feb 10, 2021

By the Pythagorean Theorem on B D E \triangle BDE , B E = B D 2 + D E 2 = ( 2 a ) 2 + a 2 = 5 a BE = \sqrt{BD^2 + DE^2} = \sqrt{(2a)^2 + a^2} = \sqrt{5}a , so tan B E D = 2 a a = 2 \tan \angle BED = \cfrac{2a}{a} = 2 and sin B E D = 2 a 5 a = 2 5 \sin \angle BED = \cfrac{2a}{\sqrt{5}a} = \cfrac{2}{\sqrt{5}} .

The area of the sector formed by E B D EBD is A sec E B D = m B E D 2 π π a 2 = 1 2 a 2 arctan ( 2 ) A_{\text{sec} EBD} = \cfrac{m\angle BED}{2\pi} \pi a^2 = \cfrac{1}{2}a^2\arctan(2) .

The area of E B D \triangle EBD is A E B D = 1 2 a 2 sin B E D = 1 5 a 2 A_{\triangle EBD} = \cfrac{1}{2}a^2 \sin \angle BED = \cfrac{1}{\sqrt{5}}a^2 .

Then A p = A sec E B D A E B D = 1 2 a 2 arctan ( 2 ) 1 5 a 2 = 1 2 A_p = A_{\text{sec} EBD} - A_{\triangle EBD} = \cfrac{1}{2}a^2\arctan(2) - \cfrac{1}{\sqrt{5}}a^2 = \cfrac{1}{2} , which solves to a 2 = 5 5 arctan ( 2 ) 2 a^2 = \cfrac{\sqrt{5}}{\sqrt{5} \arctan (2) - 2} .

And A R = A F C D E A F B E A sec E B D = a 2 1 4 a 2 1 2 a 2 arctan ( 2 ) = 3 2 arctan ( 2 ) 4 5 5 arctan ( 2 ) 2 = 5 2 2 ( 3 2 arctan ( 2 ) 5 arctan ( 2 ) 2 ) A_R = A_{FCDE} - A_{\triangle FBE} - A_{\text{sec} EBD} = a^2 - \cfrac{1}{4}a^2 - \cfrac{1}{2}a^2\arctan(2) = \cfrac{3 - 2 \arctan(2)}{4} \cdot \cfrac{\sqrt{5}}{\sqrt{5} \arctan (2) - 2} = \cfrac{\sqrt{5}}{2^2}\bigg(\cfrac{3 - 2 \arctan (2)}{\sqrt{5} \arctan (2) - 2}\bigg) .

Therefore, α = 5 \alpha = 5 , β = 2 \beta = 2 , ω = 3 \omega = 3 , and α + β + ω = 10 \alpha + \beta + \omega = \boxed{10} .

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...