Can you tell the difference?

Geometry Level 4

In the figure, four circles are inscribed in a square. The circles are tangent to each other as well as to the sides of the square and to the black line segment. The two red circles are identical. Compare the b l u e {\color{cornflowerblue}{\rm blue}} area with the g r e e n {\color{#20A900}{\rm green}} area.

Blue = Green {\color{cornflowerblue}\text{ Blue}} = {\color{#20A900}\text{ Green }} Blue < Green {\color{cornflowerblue}\text{ Blue}} < {\color{#20A900}\text{ Green }} Blue > Green {\color{cornflowerblue}\text{ Blue}} > {\color{#20A900}\text{ Green }}

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1 solution

Chew-Seong Cheong
Dec 28, 2020

Let the side length of the square be 1 1 , the radii of the white, yellow and red circles be r 1 r_1 , r 2 r_2 , and r 3 r_3 respectively.

We note the side length of the square is given by:

r 3 + ( r 1 + r 3 ) 2 ( r 1 r 3 ) 2 + r 1 = 1 r 3 + 2 r 3 r 1 + r 1 = 1 ( r 3 + r 1 ) 2 = 1 r 3 + r 1 = 1 . . . ( 1 ) \begin{aligned} r_3 + \sqrt{(r_1+r_3)^2-(r_1-r_3)^2} + r_1 & = 1 \\ r_3 + 2 \sqrt{r_3r_1} + r_1 & = 1 \\ (\sqrt{r_3}+\sqrt{r_1})^2 & = 1 \\ \implies \sqrt{r_3}+\sqrt{r_1} & = 1 & ...(1) \end{aligned}

The diagonal of the square is given by:

( 2 + 1 ) ( r 2 + r 1 ) = 2 r 2 = 2 2 r 1 . . . ( 2 ) \begin{aligned} (\sqrt 2+1)(r_2+r_1) & = \sqrt 2 \\ \implies r_2 & = 2 - \sqrt 2 - r_1 & ...(2) \end{aligned}

The length of the black line segment is given by both = 2 r 3 cot 13 5 2 + 4 r 3 r 1 \ell = 2r_3 \cot \frac {135^\circ}2 + 4 \sqrt{r_3r_1} and = 2 r 2 cot 4 5 2 \ell = 2 r_2 \cot \frac {45^\circ}2 . Therefore, we have:

r 3 cot 13 5 2 + 2 r 3 r 1 = r 2 cot 4 5 2 Note that r 3 + 2 r 3 r 1 + r 1 = 1 ( 2 1 ) r 3 + 1 r 3 r 1 = ( 2 + 1 ) r 2 ( 2 2 ) r 3 + 1 r 1 + ( 2 + 1 ) r 1 = ( 2 + 1 ) ( r 2 + r 1 ) and ( 2 + 1 ) ( r 2 + r 1 ) = 2 ( 2 2 ) r 3 + 1 + 2 r 1 = 2 and r 3 + r 1 = 1 ( 2 2 ) ( 1 r 1 ) 2 + 1 + 2 r 1 = 2 ( 2 2 ) ( 1 2 r 1 + r 1 ) + 1 + 2 r 1 = 2 2 ( 2 1 ) r 1 + 2 ( 2 2 ) r 1 1 = 0 2 r 1 + 2 2 r 1 2 1 = 0 r 1 = 3 2 + 1 2 2 + 2 0.359347716164 r 2 = 1 2 3 2 + 2 + 2 0.226438721463 From (2) r 3 = 5 2 + 1 2 2 + 2 2 ( 3 + 2 2 2 + 2 ) 0.160435348784 From (1) \begin{aligned} r_3 \cot \frac {135^\circ}2 + \blue{2 \sqrt{r_3r_1}} & = r_2 \cot \frac {45^\circ}2 & \small \blue{\text{Note that }r_3 + 2 \sqrt{r_3r_1} + r_1 = 1} \\ (\sqrt 2-1)r_3 + \blue{1-r_3-r_1} & = (\sqrt 2 + 1)r_2 \\ (\sqrt 2-2) r_3 + 1 - r_1 + \blue{(\sqrt 2+1)r_1} & = \blue{(\sqrt 2 +1)(r_2 + r_1)} & \small \blue{\text{and }(\sqrt 2+1)(r_2+r_1) = \sqrt 2} \\ (\sqrt 2-2) \blue{r_3} + 1 + \sqrt 2 r_1 & = \sqrt 2 & \small \blue{\text{and }\sqrt{r_3}+\sqrt{r_1} = 1 } \\ (\sqrt 2-2) \blue{(1-\sqrt{r_1})^2} + 1 + \sqrt 2 r_1 & = \sqrt 2 \\ (\sqrt 2-2) (1-2\sqrt{r_1}+r_1) + 1 + \sqrt 2 r_1 & = \sqrt 2 \\ 2(\sqrt 2 -1)r_1 + 2(2-\sqrt 2)\sqrt{r_1} - 1 & = 0 \\ \implies 2r_1 + 2\sqrt {2r_1} - \sqrt 2 - 1 & = 0 \\ \implies r_1 = \frac 32 + \frac 1{\sqrt 2} - \sqrt{2+\sqrt 2} & \approx 0.359347716164 \\ r_2 = \frac 12 - \frac 3{\sqrt 2} + \sqrt{2+\sqrt 2} & \approx 0.226438721463 & \small \blue{\text{From (2)}} \\ r_3 = \small \frac 52 + \frac 1{\sqrt 2} - \sqrt{2 + \sqrt 2} - \sqrt{2\left(3+\sqrt 2 - 2\sqrt{2+\sqrt 2}\right)} & \approx 0.160435348784 & \small \blue{\text{From (1)}} \end{aligned}

Note that the area of the blue triangle is Δ b l u e = ( 2 + 1 ) 2 r 2 2 \Delta_\blue{\rm blue} = (\sqrt 2+1)^2r_2^2 and then the blue area A b l u e = Δ b l u e π r 2 2 0.137766079523 A_\blue{\rm blue} = \Delta_\blue{\rm blue} - \pi r_2^2 \approx 0.137766079523 and the green area A g r e e n = 1 Δ b l u e π ( r 1 2 + 2 r 3 2 ) 0.133747976327 A_\green{\rm green} = 1 - \Delta_\blue{\rm blue} - \pi(r_1^2 + 2r_3^2) \approx 0.133747976327 . Therefore

A b l u e > A g r e e n \large \boxed{A_\blue{\rm blue} > A_\green{\rm green}}

Can you help me understand the geometry behind:

r 3 + ( r 1 + r 3 ) 2 ( r 1 r 3 ) 2 + r 1 = 1 r_{3} + \sqrt{(r_{1}+r_{3})^2-(r_{1}-r_{3})^2} + r_{1} = 1

Diogo Marques - 5 months, 2 weeks ago

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Thanos Petropoulos - 5 months, 1 week ago

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Thanks. I hated to draw the figure.

Chew-Seong Cheong - 5 months, 1 week ago

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@Chew-Seong Cheong Thank you for posting your solution. Happy new year!

Thanos Petropoulos - 5 months, 1 week ago

Thank you both.

Diogo Marques - 5 months, 1 week ago

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