More Circles!

Geometry Level pending

Diagram 1:

Diagram 2:

Extend both diagrams above to an infinite number of circles.

(Diagram 1:) In equilateral A B C \triangle{ABC} , the largest circle O 1 O_{1} is tangent to each side of A B C \triangle{ABC} and for n 2 n \geq 2 circle O n O_{n} is tangent to circle O n 1 O_{n - 1} and to A B \overline{AB} and B C \overline{BC} .

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

(Diagram 2:) For each integer n 1 n \geq 1 , circle w n w_{n} is tangent to w n 1 w_{n - 1} and tangent to the line y = x y = x and the positive x x axis.

Let C C and A A be the total circumference and area of all the circles respectively.

If S A A B C A C 2 = α β λ β β + β β \dfrac{S}{A_{\triangle{ABC}}} * \dfrac{A}{C^2} = \dfrac{\sqrt{\alpha}}{\beta^{\lambda}\sqrt{\beta^{\beta} + \beta\sqrt{\beta}}} , where α , β \alpha, \beta and λ \lambda are coprome positive integers, find α + β + λ \alpha + \beta + \lambda .


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.

1 solution

Rocco Dalto
Jan 27, 2021

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 \implies \boxed{\dfrac{S}{A_{\triangle{ABC}}} = \dfrac{\sqrt{3}}{8}\pi} .

Note: I'm using the same notation R n R_{n} in Diagram 2 to denote different radii.

O w 0 = 4 + 2 2 R 1 \overline{Ow_{0}} = \sqrt{4 + 2\sqrt{2}}R_{1}

O A 1 w 0 w 1 A 2 w 0 4 + 2 2 R 1 R 1 = R 1 + R 2 R 1 R 2 \triangle{OA_{1}w_{0}} \sim \triangle{w_{1}A_{2}w_{0}} \implies \dfrac{\sqrt{4 + 2\sqrt{2}}R_{1}}{R_{1}} = \dfrac{R_{1} + R_{2}}{R_{1} - R_{2}} \implies

( 4 + 2 2 1 ) R 1 = ( 4 + 2 2 + 1 ) R 2 (\sqrt{4 + 2\sqrt{2}} - 1)R_{1} = (\sqrt{4 + 2\sqrt{2}} + 1)R_{2} \implies R 2 = 4 + 2 2 1 4 + 2 2 + 1 R 1 R_{2} = \dfrac{\sqrt{4 + 2\sqrt{2}} - 1}{\sqrt{4 + 2\sqrt{2}} + 1}R_{1}

R 3 = 4 + 2 2 1 4 + 2 2 + 1 R 2 = ( 4 + 2 2 1 4 + 2 2 + 1 ) 2 R 1 R_{3} = \dfrac{\sqrt{4 + 2\sqrt{2}} - 1}{\sqrt{4 + 2\sqrt{2}} + 1}R_{2} = (\dfrac{\sqrt{4 + 2\sqrt{2}} - 1}{\sqrt{4 + 2\sqrt{2}} + 1})^2R_{1}

In General: R n = ( 4 + 2 2 1 4 + 2 2 + 1 ) n 1 R 1 R_{n} = (\dfrac{\sqrt{4 + 2\sqrt{2}} - 1}{\sqrt{4 + 2\sqrt{2}} + 1})^{n - 1}R_{1}

A n = π R n 2 = π R 1 2 ( ( 4 + 2 2 1 4 + 2 2 + 1 ) 2 ) n 1 \implies A_{n} = \pi R_{n}^2 = \pi R_{1}^2((\dfrac{\sqrt{4 + 2\sqrt{2}} - 1}{\sqrt{4 + 2\sqrt{2}} + 1})^2)^{n - 1}

A = n = 1 A n = π R 1 2 n = 1 ( ( 4 + 2 2 1 4 + 2 2 + 1 ) 2 ) n 1 \implies A = \sum_{n = 1}^{\infty} A_{n} = \pi R_{1}^2\sum_{n = 1}^{\infty} ((\dfrac{\sqrt{4 + 2\sqrt{2}} - 1}{\sqrt{4 + 2\sqrt{2}} + 1})^2)^{n - 1}

= ( 4 + 2 2 + 1 ) 2 4 4 + 2 2 π R 1 2 = \dfrac{(\sqrt{4 + 2\sqrt{2}} + 1)^2}{4\sqrt{4 + 2\sqrt{2}}}\pi R_{1}^2

and

C n = 2 π R n = 2 π R 1 ( 4 + 2 2 1 4 + 2 2 + 1 ) n 1 C_{n} = 2\pi R_{n} = 2\pi R_{1}(\dfrac{\sqrt{4 + 2\sqrt{2}} - 1}{\sqrt{4 + 2\sqrt{2}} + 1})^{n - 1}

C = n = 1 C n = 2 π R 1 n = 1 ( 4 + 2 2 1 4 + 2 2 + 1 ) n 1 = \implies C = \sum_{n = 1}^{\infty} C_{n} = 2\pi R_{1}\sum_{n = 1}^{\infty} (\dfrac{\sqrt{4 + 2\sqrt{2}} - 1}{\sqrt{4 + 2\sqrt{2}} + 1})^{n - 1} =

( 4 + 2 2 ) π R 1 C 2 = ( 4 + 2 2 ) 2 π 2 R 1 2 (\sqrt{4 + 2\sqrt{2}}) \pi R_{1} \implies C^2 = (\sqrt{4 + 2\sqrt{2}})^2 \pi^2 R_{1}^2

A C 2 = 1 4 π 4 + 2 2 \implies \boxed{\dfrac{A}{C^2} = \dfrac{1}{4\pi\sqrt{4 + 2\sqrt{2}}}}

S A A B C A C 2 = 3 2 5 2 2 + 2 2 = \implies \dfrac{S}{A_{\triangle{ABC}}} * \dfrac{A}{C^2} = \dfrac{\sqrt{3}}{2^5\sqrt{2^2 + 2\sqrt{2}}} = α β λ β β + β β \dfrac{\sqrt{\alpha}}{\beta^{\lambda}\sqrt{\beta^{\beta} + \beta\sqrt{\beta}}} \implies

α + β + λ = 10 \alpha + \beta + \lambda = \boxed{10} .

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...