Circle Mania 2.

Geometry Level pending

In the diagram above square A B C D ABCD has side length 1 1 and the pink, red and blue circle are tangent to each other and tangent to the green circle and B C \overline{BC} is tangent to the pink circle and blue circle and A B \overline{AB} is tangent to the pink and red circle.

Let S S be the sum of the areas of the pink, red and blue circles.

If S = α β π ( λ ω β ) S = \dfrac{\alpha}{\beta}\pi(\lambda - \omega\sqrt{\beta}) , where α , β , λ \alpha, \beta, \lambda and ω \omega are coprime positive integers, find α + β + λ + ω \alpha + \beta + \lambda + \omega .


The answer is 34.

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3 solutions

Rocco Dalto
Jan 24, 2021

Note: We just need to find R R then r r since r = r r = r' using the symmetry about B D \overline{BD} .

B D = 2 = 1 + R + 2 R = 1 + ( 2 + 1 ) R R = 2 1 2 + 1 = 3 2 2 \overline{BD} = \sqrt{2} = 1 + R + \sqrt{2}R = 1 + (\sqrt{2} + 1)R \implies R = \dfrac{\sqrt{2} - 1}{\sqrt{2} + 1} = 3 - 2\sqrt{2}

Using O E O m 2 = ( R + r ) 2 ( R r ) 2 = 4 R r m = 2 R r \triangle{O'EO} \implies m^2 = (R + r)^2 - (R - r)^2 = 4Rr \implies m = 2\sqrt{Rr}

and

Using F O D ( 1 + r ) 2 = ( 1 r ) 2 + ( 1 R m ) 2 \triangle{FO'D} \implies (1 + r)^2 = (1- r)^2 + (1 - R - m)^2 \implies

4 r = ( 1 R ) 2 2 m ( 1 r ) + m 2 = ( 1 R ) 2 4 ( 1 R ) R r + 4 R r 4r = (1 - R)^2 - 2m(1 - r) + m^2 = (1 - R)^2 - 4(1 - R)\sqrt{Rr} + 4Rr \implies

4 ( 1 R ) r = ( 1 R ) 2 4 R r ( 1 R ) 4 r = 1 R 4 R r 4(1 - R)r = (1 - R)^2 - 4\sqrt{Rr}(1 - R) \implies 4r = 1 - R - 4\sqrt{Rr} \implies

4 r + 4 R r + R = 1 ( R + 2 r ) 2 = 1 R + 2 r = 1 4r + 4\sqrt{Rr} + R = 1 \implies (\sqrt{R} + 2\sqrt{r})^2 = 1 \implies \sqrt{R} + 2\sqrt{r} = 1 \implies

r = 1 R 2 = 1 3 2 2 2 = 1 ( 2 1 ) 2 2 = 2 2 2 = 2 ( 2 1 ) 2 \sqrt{r} = \dfrac{1 - \sqrt{R}}{2} = \dfrac{1 - \sqrt{3 - 2\sqrt{2}}}{2} = \dfrac{1 - \sqrt{(\sqrt{2} - 1})^2}{2} = \dfrac{2 - \sqrt{2}}{2} = \dfrac{\sqrt{2}(\sqrt{2} - 1)}{2}

r = 2 ( 3 2 2 ) 4 = 3 2 2 2 = R 2 S = π R 2 + 2 π ( R 2 ) 2 = \implies r = \dfrac{2(3 - 2\sqrt{2})}{4} = \dfrac{3 - 2\sqrt{2}}{2} = \dfrac{R}{2} \implies S = \pi R^2 + 2\pi(\dfrac{R}{2})^2 =

3 2 π ( 3 2 2 ) 2 = \dfrac{3}{2}\pi(3 - 2\sqrt{2})^2 = 3 2 π ( 17 12 2 ) = α β π ( λ ω β ) α + β + λ + ω = 34 \dfrac{3}{2}\pi(17 - 12\sqrt{2}) = \dfrac{\alpha}{\beta}\pi(\lambda - \omega\sqrt{\beta}) \implies \alpha + \beta + \lambda + \omega = \boxed{34} .

David Vreken
Jan 25, 2021

Let the radius of the pink circle be R R and the radius of the red and blue circles be r r . Add E E and F F as follows:

As a diagonal of a unit square, B D = 2 BD = \sqrt{2} . As a hypotenuse of right B F O \triangle BFO with O F = R OF = R , B O = 2 R BO = \sqrt{2}R . Therefore, B D = B O + O E + E D = 2 R + R + 1 = 2 BD = BO + OE + ED = \sqrt{2}R + R + 1 = \sqrt{2} , which solves to R = 3 2 2 R = 3 - 2\sqrt{2} .

By Descartes' Theorem (special case with a straight line), 1 r = 1 + 1 R + 2 1 1 R = 1 + 1 3 2 2 + 2 1 1 3 2 2 = 2 3 2 2 \cfrac{1}{r} = 1 + \cfrac{1}{R} + 2\sqrt{1 \cdot \cfrac{1}{R}} = 1 + \cfrac{1}{3 - 2\sqrt{2}} + 2\sqrt{1 \cdot \cfrac{1}{3 - 2\sqrt{2}}} = \cfrac{2}{3 - 2\sqrt{2}} , so that r = 1 2 ( 3 2 2 ) r = \frac{1}{2}(3 - 2\sqrt{2}) .

Therefore, S = π R 2 + 2 π r 2 = π ( ( 3 2 2 ) 2 + 2 ( 1 2 ( 3 2 2 ) ) 2 ) = 3 2 ( 17 12 2 ) S = \pi R^2 + 2\pi r^2 = \pi((3 - 2\sqrt{2})^2 + 2(\frac{1}{2}(3 - 2\sqrt{2}))^2) = \frac{3}{2}(17 - 12\sqrt{2}) , so α = 3 \alpha = 3 , β = 2 \beta = 2 , λ = 17 \lambda = 17 , ω = 12 \omega = 12 , and α + β + λ + ω = 34 \alpha + \beta + \lambda + \omega = \boxed{34} .

Hongqi Wang
Jan 25, 2021

r p i n k = 3 2 2 r r e d = r b l u e = 3 2 2 2 = r p i n k 2 S = π ( r p i n k 2 + r r e d 2 + r b l u e 2 ) = 3 2 π r p i n k 2 = 3 2 π ( 3 2 2 ) 2 = 3 2 π ( 17 12 2 ) r_{pink} = 3 - 2 \sqrt 2 \\ r_{red} = r_{blue} = \dfrac {3 - 2 \sqrt 2}2 = \dfrac {r_{pink}}2 \\ S = \pi (r_{pink}^2 + r_{red}^2 + r_{blue}^2) \\ = \dfrac 32 \pi r_{pink}^2 \\ = \dfrac 32 \pi (3 - 2 \sqrt 2)^2 \\ = \dfrac 32 \pi (17 - 12 \sqrt 2)

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