What's the Radius?

Geometry Level 4

In square A B C D ABCD with side length 1 1 and diagonal A C \overline{AC} and E C \overline{EC} , the two red circles are congruent with radius r and tangent to the sides of A C E \triangle{ACE} and E C D \triangle{ECD} as shown above.

If the radius r r can be expressed as r = λ ω λ ω r = \dfrac{\lambda - \sqrt{\sqrt{\omega} - \lambda}}{\omega} , where λ \lambda and ω \omega are coprime positive integers, find λ + ω \lambda + \omega .


The answer is 3.

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3 solutions

Let the centers of the left and right circles be O O and P P respectively, O M OM and P N PN be perpendicular to A D AD , P L PL is perpendicular to C D CD , and E C D = θ \angle ECD = \theta . Then C E D = 9 0 θ \angle CED = 90^\circ - \theta and C E A = 9 0 + θ \angle CEA = 90^\circ + \theta . Note that C A D = 4 5 \angle CAD = 45^\circ , and C P CP , E P EP , O E OE , and A O AO bisect E C D \angle ECD , C E D \angle CED , C E A \angle CEA , and C A D \angle CAD respectively.

We can apply half-angle tangent substitution to solve triangle incircle problems reducing them to solving polynomial equations. From

C L + L D = C D r cot θ 2 + r = 1 Let t = tan θ 2 r t + r = 1 t = r 1 r \begin{aligned} CL + LD & = CD \\ r \cot \frac \theta 2 + r & = 1 & \small \blue{\text{Let }t = \tan \frac \theta 2} \\ \frac rt + r & = 1 \\ \implies t & = \frac r{1-r} \end{aligned}

From:

A M + M E + E N + N D = A D r cot 22. 5 + r cot ( 4 5 + θ 2 ) + r cot ( 4 5 θ 2 ) + r = 1 ( 2 + 1 ) r + 1 t 1 + t r + 1 + t 1 t r + r = 1 Note that t = r 1 r ( 2 + 2 ) r + ( 1 2 r ) r + r 1 2 r = 1 4 r 3 ( 8 + 2 2 ) r 2 + ( 6 + 2 ) r 1 = 0 for r 1 2 r = { 1 + 1 2 1 2 1 2 1 + 2 1 2 \begin{aligned} AM + ME + EN + ND & = AD \\ r\cot 22.5^\circ + r \cot \left(45^\circ + \frac \theta 2 \right) + r \cot \left(45^\circ - \frac \theta 2 \right) + r & = 1 \\ (\sqrt 2 + 1)r + \frac {1-t}{1+t} \cdot r + \frac {1+t}{1-t} \cdot r + r & = 1 & \small \blue{\text{Note that }t = \frac r{1-r}} \\ (\sqrt 2 + 2)r + (1-2r)r + \frac r{1-2r} & = 1 \\ 4r^3 - (8+2\sqrt 2)r^2 + (6+\sqrt 2)r - 1 & = 0 & \small \blue{\text{for }r \ne \frac 12} \\ \implies r & = \begin{cases} 1 + \dfrac 1{\sqrt 2} \\ \dfrac {1-\sqrt{\sqrt 2 -1}}2 \\ \dfrac {1+\sqrt{\sqrt 2 -1}}2 \end{cases} \end{aligned}

The acceptable value is r = 1 2 1 2 r = \dfrac {1-\sqrt{\sqrt 2-1}}2 . Therefore λ + ω = 1 + 2 = 3 \lambda + \omega = 1+2 = \boxed 3 .

Rocco Dalto
Mar 7, 2021

A E C D = 1 2 y = 1 2 r ( a + y + 1 ) r = y a + y + 1 A_{\triangle{ECD}} = \dfrac{1}{2}y = \dfrac{1}{2}r(a + y + 1) \implies r = \dfrac{y}{a + y + 1}

and

A A C E = 1 2 r ( a + 2 + 1 y ) = 1 2 ( 1 y ) A_{\triangle{ACE}} = \dfrac{1}{2}r(a + \sqrt{2} + 1 - y) = \dfrac{1}{2}(1 - y) \implies

r = 1 y a + 2 + 1 y = y a + y + 1 ( 2 a + 2 + 1 ) y = a + 1 r = \dfrac{1 - y}{a + \sqrt{2} + 1 - y} = \dfrac{y}{a + y + 1} \implies (2a + \sqrt{2} + 1)y = a + 1 \implies

y = a + 1 2 a + 2 + 1 y = \dfrac{a + 1}{2a + \sqrt{2} + 1}

In E C D y 2 + 1 = a 2 ( a + 1 ) 2 + ( 2 a + 2 + 1 ) 2 = a 2 ( 2 a + 2 + 1 ) 2 \triangle{ECD} \implies y^2 + 1 = a^2 \implies (a + 1)^2 + (2a + \sqrt{2} + 1)^2 = a^2(2a + \sqrt{2} + 1)^2

( a + 1 ) ( a + 1 ( a 1 ) ( 2 a + 2 + 1 ) 2 ) = 0 \implies (a + 1)(a + 1 - (a - 1)(2a + \sqrt{2} + 1)^2) = 0 and a 1 a \neq 1 \implies

2 a 3 + 2 2 a 2 ( 2 + 1 ) a 2 ( 2 + 1 ) = 0 2a^3 + 2\sqrt{2}a^2 - (\sqrt{2} + 1)a - \sqrt{2}(\sqrt{2} + 1) = 0 \implies

2 a 2 ( a + 2 ) ( 2 + 1 ) ( a + 2 ) = 0 ( 2 a 2 ( 2 + 1 ) ) ( a + 2 ) = 0 2a^2(a + \sqrt{2}) - (\sqrt{2} + 1)(a + \sqrt{2}) = 0 \implies (2a^2 - (\sqrt{2} + 1))(a + \sqrt{2}) = 0

and a 2 a 2 = 2 + 1 2 a = 2 + 1 2 a \neq -\sqrt{2} \implies a^2 = \dfrac{\sqrt{2} + 1}{2} \implies \boxed{a = \sqrt{\dfrac{\sqrt{2} + 1}{2}}}

y 2 + 1 = 2 + 1 2 y = 2 1 2 \implies y^2 + 1 = \dfrac{\sqrt{2} + 1}{2} \implies \boxed{y = \sqrt{\dfrac{\sqrt{2} - 1}{2}}}

Let α = 2 + 1 \alpha = \sqrt{2} + 1 and β = 2 1 \beta = \sqrt{2} - 1 a = α 2 \implies a = \dfrac{\sqrt{\alpha}}{\sqrt{2}} and y = β 2 y = \dfrac{\sqrt{\beta}}{\sqrt{2}} \implies

r = y a + y + 1 = β α + β + 2 = r = \dfrac{y}{a + y + 1} = \dfrac{\sqrt{\beta}}{\sqrt{\alpha} + \sqrt{\beta} + \sqrt{2}} = 2 β ( α + β 2 ) 4 \dfrac{\sqrt{2}\sqrt{\beta}(\sqrt{\alpha} + \sqrt{\beta} - \sqrt{2})}{4}

= 2 ( 1 + 2 1 2 β ) 4 = 2 ( 1 β ) 4 = = \dfrac{\sqrt{2}(1 + \sqrt{2} - 1 - \sqrt{2}\sqrt{\beta})}{4} = \dfrac{2(1 - \sqrt{\beta})}{4} =

1 2 1 2 = λ ω λ ω λ + ω = 3 \dfrac{1 - \sqrt{\sqrt{2} - 1}}{2} = \dfrac{\lambda - \sqrt{\sqrt{\omega} - \lambda}}{\omega} \implies \lambda + \omega = \boxed{3} .

David Vreken
Mar 8, 2021

By the properties of a unit square, A D = C D = 1 AD = CD = 1 and A C = 2 AC = \sqrt{2} . Let E D = x ED = x , so that A E = A D E D = 1 x AE = AD - ED = 1 - x , and E C = E D 2 + C D 2 = x 2 + 1 EC = \sqrt{ED^2 + CD^2} = \sqrt{x^2 + 1} .

Then the areas of A E C \triangle AEC and E D C \triangle EDC are A A E C = 1 2 A E C D = 1 2 ( 1 x ) 1 = 1 2 ( 1 x ) A_{\triangle AEC} = \frac{1}{2} \cdot AE \cdot CD = \frac{1}{2} \cdot (1 - x) \cdot 1 = \frac{1}{2}(1 - x) and A E D C = 1 2 E D C D = 1 2 x 1 = 1 2 x A_{\triangle EDC} = \frac{1}{2} \cdot ED \cdot CD = \frac{1}{2} \cdot x \cdot 1 = \frac{1}{2}x .

And the perimeters are P A E C = A E + A C + E C = 1 x + 2 + x 2 + 1 P_{\triangle AEC} = AE + AC + EC = 1 - x + \sqrt{2} + \sqrt{x^2 + 1} and P E D C = E D + C D + E C = x + 1 + x 2 + 1 P_{\triangle EDC} = ED + CD + EC = x + 1 + \sqrt{x^2 + 1} .

Since the inradius is r = 2 A P r = \cfrac{2A}{P} , then r = 2 A A E C P A E C = 2 A E D C P E D C r = \cfrac{2A_{\triangle AEC}}{P_{\triangle AEC}} = \cfrac{2A_{\triangle EDC}}{P_{\triangle EDC}} , or r = 1 x 1 x + 2 + x 2 + 1 = x x + 1 + x 2 + 1 r = \cfrac{1 - x}{1 - x + \sqrt{2} + \sqrt{x^2 + 1}} = \cfrac{x}{x + 1 + \sqrt{x^2 + 1}} , which solves to x = 1 2 ( 2 1 ) x = \sqrt{\frac{1}{2}(\sqrt{2} - 1)} .

Substituting x = 1 2 ( 2 1 ) x = \sqrt{\frac{1}{2}(\sqrt{2} - 1)} into r = x x + 1 + x 2 + 1 r = \cfrac{x}{x + 1 + \sqrt{x^2 + 1}} and solving gives r = 1 2 1 2 r = \cfrac{1 - \sqrt{\sqrt{2} - 1}}{2} , so λ = 1 \lambda = 1 , ω = 2 \omega = 2 , and λ + ω = 3 \lambda + \omega = \boxed{3} .

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