is a regular pentagon with side length . You construct isosceles triangle 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 of each of these three circles, and submit .
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.
Let the centers of center and left circles be O and P respectively, and O R and P S be perpendicular to A B and E A respectively. Let ∠ F A B = 2 θ . Then we note that tan θ = A R O R = 4 r and:
E S + S A P S cot ∠ P E S + P S cot ∠ P A S r cot 5 4 ∘ + r cot ( 5 4 ∘ − θ ) tan 3 6 ∘ + tan ( 3 6 ∘ + θ ) ⟹ tan θ r ⟹ ⌊ 1 0 4 r ⌋ = E A = E A = 8 = r 8 = tan θ 2 ≈ 0 . 6 2 4 7 7 5 6 3 8 = 4 tan θ ≈ 2 . 4 9 9 1 0 2 5 5 = 2 4 9 9 1