Circles inscribed in equilateral triangle!

Level pending

In equilateral A B C \triangle{ABC} above, extend the diagram to an infinite number of inscribed circles.

Let S S be the total area of all the circles.

If S A A B C = α β π \dfrac{S}{A_{\triangle{ABC}}} = \dfrac{\sqrt{\alpha}}{\beta}\pi , where α \alpha and β \beta are coprime positive integers, find α + β \alpha + \beta .


The answer is 11.

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1 solution

Rocco Dalto
Oct 21, 2020

Using the above diagram we obtain:

2 R 1 a 1 = tan ( 3 0 ) = 1 3 R 1 = a 1 2 3 \dfrac{2R_{1}}{a_{1}} = \tan(30^{\circ}) = \dfrac{1}{\sqrt{3}} \implies \boxed{R_{1} = \dfrac{a_{1}}{2\sqrt{3}}}

and H 2 = H 1 2 R 1 = 3 2 a 1 1 3 a 1 = a 1 2 3 H_{2} = H_{1} - 2R_{1} = \dfrac{\sqrt{3}}{2}a_{1} - \dfrac{1}{\sqrt{3}}a_{1} = \dfrac{a_{1}}{2\sqrt{3}}

H 1 H 2 = 3 = a 1 a 2 a 2 = a 3 3 \dfrac{H_{1}}{H_{2}} = 3 = \dfrac{a_{1}}{a_{2}} \implies a_{2} = \dfrac{a_{3}}{3}

2 R 2 a 2 = 1 3 R 2 = a 2 2 3 = a 1 6 3 \dfrac{2R_{2}}{a_{2}} = \dfrac{1}{\sqrt{3}} \implies \boxed{R_{2} = \dfrac{a_{2}}{2\sqrt{3}} = \dfrac{a_{1}}{6\sqrt{3}}}

H 3 = H 2 2 R 2 = a 1 2 3 a 1 3 3 = 3 18 a 1 H_{3} = H_{2} - 2R_{2} = \dfrac{a_{1}}{2\sqrt{3}} - \dfrac{a_{1}}{3\sqrt{3}} = \dfrac{\sqrt{3}}{18}a_{1} = a 1 6 3 = \dfrac{a_{1}}{6\sqrt{3}}

H 2 H 3 = 3 = a 2 a 3 a 3 = a 2 3 = a 1 9 \dfrac{H_{2}}{H_{3}} = 3 = \dfrac{a_{2}}{a_{3}} \implies a_{3} = \dfrac{a_{2}}{3} = \dfrac{a_{1}}{9} \implies

2 R 3 a 3 = 1 3 R 3 = a 3 2 3 = a 1 18 3 \dfrac{2R_{3}}{a_{3}} = \dfrac{1}{\sqrt{3}} \implies \boxed{R_{3} = \dfrac{a_{3}}{2\sqrt{3}} = \dfrac{a_{1}}{18\sqrt{3}}}

In General:

R n = a 1 2 3 ( 1 3 ) n 1 R_{n} = \dfrac{a_{1}}{2\sqrt{3}}(\dfrac{1}{3})^{n - 1}

Note: H n = a 1 2 3 ( 1 3 ) n 2 H_{n} = \dfrac{a_{1}}{2\sqrt{3}}(\dfrac{1}{3})^{n - 2}

A n = π R n 2 = π 12 ( 1 9 ) n 1 a 1 2 \implies A_{n} = \pi R_{n}^2 = \dfrac{\pi}{12}(\dfrac{1}{9})^{n - 1}a_{1}^2

S = n = 1 A n = π 12 a 1 2 ( 9 8 ) = 3 32 π a 1 2 \implies S = \sum_{n = 1}^{\infty} A_{n} = \dfrac{\pi}{12}a_{1}^2(\dfrac{9}{8}) = \dfrac{3}{32}\pi a_{1}^2 = 3 4 ( 3 π 8 ( 1 3 ) a 1 2 = 3 π 8 3 A A B C = = \dfrac{\sqrt{3}}{4}(\dfrac{3\pi}{8}(\dfrac{1}{\sqrt{3}})a_{1}^2 = \dfrac{3\pi}{8\sqrt{3}}A_{\triangle{ABC}} = 3 8 π A A B C \dfrac{\sqrt{3}}{8}\pi A_{\triangle{ABC}}

S A A B C = 3 8 π = α β π \implies \dfrac{S}{A_{\triangle{ABC}}} = \dfrac{\sqrt{3}}{8}\pi = \dfrac{\sqrt{\alpha}}{\beta}\pi α + β = 11 \implies \alpha + \beta = \boxed{11} .

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