Infinite Tangent Circles

Geometry Level pending

Let A , B , C A, B, C be points on the same plane with A C B = 12 0 \angle ACB=120^{\circ} . There is a sequence of circles ω 0 , ω 1 , ω 2 , . . . \omega_0, \omega_1, \omega_2, ... on the same plane (with corresponding radii r 0 , r 1 , r 2 , . . . r_0, r_1, r_2, ... , where r 0 > r 1 > r 2 > . . . r_0>r_1>r_2>... ) such that each circle is tangent to both segments C A CA and C B CB . Furthermore, ω i \omega_i is tangent to ω i 1 \omega_{i-1} for all i 1 i\ge1 . If r 0 = 3 r_0=3 , find the value of r 0 + r 1 + r 2 + . . . r_0+r_1+r_2+...


The answer is 3.232050808.

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1 solution

Michael Mendrin
Jun 20, 2018

The distance from C to the far side of the circle of radius 3 3 is 3 + 3 2 3 3+3\frac{2}{\sqrt{3}} , so the sum of the radii is half that, or

3 2 + 3 3.23205 \dfrac{3}{2}+\sqrt{3} \approx 3.23205

The ratio between successive radii is 7 4 3 7-4\sqrt{3} , but is not needed.

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