Three circles in a pentagon

Geometry Level 4

Fig. 1 Three circles inscribed in a pentagon Fig. 1 Three circles inscribed in a pentagon

A B C D E ABCDE is a regular pentagon with side length 8 8 . You construct isosceles triangle F A B FAB such that its incircle and the two circles tangent to it and to the adjacent pentagon sides all have the same radius, as shown in the figure above. Find the radius r r of each of these three circles, and submit 1 0 4 r \lfloor 10^4 r \rfloor .


The answer is 24991.

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1 solution

Chew-Seong Cheong
Dec 29, 2020

Let the centers of center and left circles be O O and P P respectively, and O R OR and P S PS be perpendicular to A B AB and E A EA respectively. Let F A B = 2 θ \angle FAB = 2\theta . Then we note that tan θ = O R A R = r 4 \tan \theta = \dfrac {OR}{AR} = \dfrac r4 and:

E S + S A = E A P S cot P E S + P S cot P A S = E A r cot 5 4 + r cot ( 5 4 θ ) = 8 tan 3 6 + tan ( 3 6 + θ ) = 8 r = 2 tan θ tan θ 0.624775638 r = 4 tan θ 2.49910255 1 0 4 r = 24991 \begin{aligned} ES + SA & = EA \\ PS \cot \angle PES + PS \cot \angle PAS & = EA \\ r \cot 54^\circ + r \cot (54^\circ - \theta) & = 8 \\ \tan 36^\circ + \tan (36^\circ + \theta) & = \frac 8r = \frac 2{\tan \theta} \\ \implies \tan \theta & \approx 0.624775638 \\ r & = 4\tan \theta \approx 2.49910255 \\ \implies \lfloor 10^4r \rfloor & = \boxed{24991} \end{aligned}

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