Circle, square and equilateral triangle inscribed in a right triangle.

Geometry Level 4

A unit circle, a square and an equilateral triangle are inscribed in right A B C \triangle{ABC} as shown above.

Let A T A_{T} be the sum of the areas of the square and the equilateral triangle.

If A T = α β + λ β A_{T} = \dfrac{\alpha\sqrt{\beta} + \lambda}{\beta} , where α , β \alpha,\beta and λ \lambda are coprime positive integers, find α + β + λ \alpha + \beta + \lambda .


The answer is 40.

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1 solution

Rocco Dalto
Feb 27, 2021

m K D I = 6 0 m K D O = 3 0 m D O K = 6 0 m\angle{KDI} = 60^{\circ} \implies m\angle{KDO} = 30^{\circ} \implies m\angle{DOK} = 60^{\circ}

For K D O : \triangle{KDO}:

l = tan ( 6 0 ) = 3 D E = 3 + 1 = E F l = \tan(60^{\circ}) = \sqrt{3} \implies \overline{DE} = \sqrt{3} + 1 = \overline{EF} \implies

The area of the square A s = ( 3 + 1 ) 2 = 4 + 2 3 \boxed{A_{s} = (\sqrt{3} + 1)^2 = 4 + 2\sqrt{3}}

For J O G : \triangle{JOG}:

m H G J = 12 0 m\angle{HGJ} = 120^{\circ} and m O J G = m O H G = 9 0 m\angle{OJG} = m\angle{OHG} = 90^{\circ} \implies m J O H = 6 0 m\angle{JOH} = 60^{\circ} in quadrilateral J O H G JOHG

m J O G = 3 0 m J G O = 6 0 1 n = tan ( 6 0 ) = 3 \implies m\angle{JOG} = 30^{\circ} \implies m\angle{JGO} = 60^{\circ} \implies \dfrac{1}{n} = \tan(60^{\circ}) = \sqrt{3} \implies

n = 1 3 = J G = H G G F = E F + E H + H G = 3 + 2 + 1 3 = n = \dfrac{1}{\sqrt{3}} = \overline{JG} = \overline{HG} \implies \overline{GF} = \overline{EF} + \overline{EH} + \overline{HG} = \sqrt{3} + 2 + \dfrac{1}{\sqrt{3}} =

4 + 2 3 3 A G F M = 3 4 ( 4 + 2 3 3 ) 2 = 7 3 + 12 3 \dfrac{4 +2\sqrt{3}}{\sqrt{3}} \implies \boxed{A_{\triangle{GFM}} = \dfrac{\sqrt{3}}{4}(\dfrac{4 + 2\sqrt{3}}{\sqrt{3}})^2 = \dfrac{7\sqrt{3} + 12}{3}} \implies

A T = 13 3 + 24 3 = α β + λ β α + β + λ = 40 A_{T} = \dfrac{13\sqrt{3} + 24}{3} = \dfrac{\alpha\sqrt{\beta} + \lambda}{\beta} \implies \alpha + \beta + \lambda = \boxed{40}

Nice problem and solution! You have a typo, "area of circle" should be "area of square".

David Vreken - 3 months, 2 weeks ago

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Thank You. I changed it to area of square.

Rocco Dalto - 3 months, 2 weeks ago

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