A circle and equilateral triangle inscribed in a semicircle.

Geometry Level 3

A circle and equilateral triangle are inscribed in a semicircle with radius 1 1 and the circle is tangent to the semicircle and equilateral triangle as shown above and one of the vertices of the equilateral triangle is at the center of the semicircle.

If the area of the circle A c = α ( β λ α ) π A_{c} = \alpha(\beta - \lambda\sqrt{\alpha})\pi , where α , β \alpha, \beta and λ \lambda are coprime positive integers, find α + β + λ \alpha + \beta + \lambda .


The answer is 14.

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2 solutions

Rocco Dalto
Feb 26, 2021

m C F D = 6 0 m A F C = 12 0 m\angle{CFD} = 60^{\circ} \implies m\angle{AFC} = 120^{\circ} and letting m N O E = θ m\angle{NOE} = \theta \implies

12 0 = 1 2 ( m ( N B E ^ ) m ( N R E ^ ) ) = 1 2 ( 36 0 2 θ ) 120^{\circ} = \dfrac{1}{2}(m(\widehat{NBE}) - m(\widehat{NRE})) = \dfrac{1}{2}(360^{\circ} - 2\theta) \implies

24 0 = 36 0 2 θ θ = 6 0 m N O F = 3 0 240^{\circ} = 360^{\circ} - 2\theta \implies \theta = 60^{\circ} \implies m\angle{NOF} =30^{\circ} \implies

O F N = 6 0 r y = tan ( 6 0 ) = 3 \angle{OFN} = 60^{\circ} \implies \dfrac{r}{y} = \tan(60^{\circ}) = \sqrt{3} , where y = N F y = \overline{NF}

y = r 3 m = 2 r 3 B F = r + m = 2 + 3 3 r = 1 \implies y = \dfrac{r}{\sqrt{3}} \implies m = \dfrac{2r}{\sqrt{3}} \implies \overline{BF} = r + m = \dfrac{2 + \sqrt{3}}{\sqrt{3}}r = 1 \implies

r = 3 2 + 3 = 2 3 3 A c = π ( 2 3 3 ) 2 = 3 ( 7 4 3 ) π r = \dfrac{\sqrt{3}}{2 + \sqrt{3}} = 2\sqrt{3} - 3 \implies A_{c} = \pi(2\sqrt{3} - 3)^2 = 3(7 - 4\sqrt{3})\pi

α ( β λ α ) π α + β + λ = 14 \alpha(\beta - \lambda\sqrt{\alpha})\pi \implies \alpha + \beta + \lambda = \boxed{14}

Are there any typos in the second line? Should A B E ABE be A B C ABC and should N R E NRE be N O E NOE ?

Fletcher Mattox - 3 months, 2 weeks ago

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Yes there is. It should be:

m C F D = 6 0 m A F C = 12 0 m\angle{CFD} = 60^{\circ} \implies m\angle{AFC} = 120^{\circ} and letting m N O E = θ m\angle{NOE} = \theta \implies

12 0 = 1 2 ( m ( N B E ^ ) m ( N R E ^ ) ) = 1 2 ( 36 0 2 θ ) 120^{\circ} = \dfrac{1}{2}(m(\widehat{NBE}) - m(\widehat{NRE})) = \dfrac{1}{2}(360^{\circ} - 2\theta) \implies

24 0 = 36 0 2 θ θ = 6 0 m N O F = 3 0 240^{\circ} = 360^{\circ} - 2\theta \implies \theta = 60^{\circ} \implies m\angle{NOF} =30^{\circ} \implies

O F N = 6 0 \angle{OFN} = 60^{\circ}

and I forgot to put the R R in the diagram for N R E ^ \widehat{NRE} . I'll add it. Thanks!

Rocco Dalto - 3 months, 2 weeks ago
Chew-Seong Cheong
Feb 27, 2021

Let the equilateral triangle be A B C ABC , where C C is the center of the semicircle, the tangent points to equilateral triangle, the semicircle diameter, and the semicircle arc be M M , N N , and Q Q respectively, and the radius of the circle be r r . Then O M = O N = O Q = r OM=ON=OQ=r , where O O is the center of the circle, and they are perpendicular to the respective line.

We note that C C , O O , and Q Q are colinear. Since A B C \triangle ABC is equilateral, A C B = 6 0 \angle ACB = 60^\circ , A C N = 18 0 6 0 = 12 0 \implies \angle ACN = 180^\circ - 60^\circ = 120^\circ . Due to symmetry, O C M = O C N = 6 0 \angle OCM = \angle OCN = 60^\circ . Then

Q O + O C = Q C Q O + O N csc O C N = Q C r + r csc 6 0 = 1 r = 1 1 + csc 6 0 = 1 1 + 2 3 = 3 3 + 2 = 3 ( 2 3 ) 4 3 = 2 3 3 A c = π r 2 = ( 2 3 3 ) 2 π = ( 21 12 3 ) π = 3 ( 7 4 3 ) π \begin{aligned} QO+OC& = QC \\ QO + ON \csc \angle OCN & = QC \\ r + r \csc 60^\circ & = 1 \\ r & = \frac 1{1+\csc 60^\circ} = \frac 1{1+\frac 2{\sqrt 3}} = \frac {\sqrt 3}{\sqrt 3 + 2} = \frac {\sqrt 3 (2-\sqrt 3)}{4-3} = 2\sqrt 3 - 3 \\ \implies A_c & = \pi r^2 = (2\sqrt 3 - 3)^2 \pi = (21 - 12\sqrt 3)\pi = 3 (7 - 4 \sqrt 3)\pi \end{aligned}

Therefore α + β + γ = 3 + 7 + 4 = 14 \alpha + \beta + \gamma = 3+7+4 = \boxed{14} .

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