Valentines Day Problem 1.

Geometry Level pending

A B C D ABCD is a square and B C \overline{BC} and C D \overline{CD} are diameters of both red semicircles and the heart formed is inscribed in the blue circle and tangent to both red circles as shown above and the blue circle intersects the heart at vertex A A of the square.

Let A c A_{c} and A s A_{s} be the areas of the blue circle and the square respectively.

If A c A s = α ( β + γ α ) ω π \dfrac{A_{c}}{A_{s}} = \dfrac{\alpha(\beta + \gamma\sqrt{\alpha})}{\omega}\pi , where α , β , γ \alpha,\beta,\gamma and ω \omega are coprime positive integers, find α + β + γ + ω \alpha + \beta + \gamma + \omega .


The answer is 68.

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2 solutions

Rocco Dalto
Feb 15, 2021

Using the diagram above and the law of cosines on C E F \triangle{CEF} with included m E C F = 4 5 m\angle{ECF} = 45^{\circ} we have:

( r a 2 ) 2 = ( 2 a r ) 2 + a 2 4 2 ( a 2 ) ( 2 a r 2 ) (r - \dfrac{a}{2})^2 = (\sqrt{2}a - r)^2 + \dfrac{a^2}{4} - 2(\dfrac{a}{2})(\dfrac{\sqrt{2}a - r}{\sqrt{2}} ) \implies

4 r 2 4 a r + a 2 = 8 a 2 8 2 a r 4 r 2 + a 2 2 2 a ( 2 a r ) 4r^2 - 4ar + a^2 = 8a^2 - 8\sqrt{2}ar - 4r^2 + a^2 - 2\sqrt{2}a(\sqrt{2}a - r) \implies

4 a r = 6 2 a r + 4 a 2 2 ( 3 2 2 ) a r = 4 a 2 r = 3 2 + 2 7 a -4ar = -6\sqrt{2}ar + 4a^2 \implies 2(3\sqrt{2} - 2)ar = 4a^2 \implies r = \dfrac{3\sqrt{2} + 2}{7}a

A c A s = ( 3 2 + 2 ) 2 49 π = 2 ( 11 + 6 2 ) 49 π = \implies \dfrac{A_{c}}{A_{s}} = \dfrac{(3\sqrt{2} + 2)^2}{49}\pi = \dfrac{2(11 + 6\sqrt{2})}{49}\pi =

α ( β + γ α ) ω π α + β + γ + ω = 68 \dfrac{\alpha(\beta + \gamma\sqrt{\alpha})}{\omega}\pi \implies \alpha + \beta + \gamma + \omega = \boxed{68} .

David Vreken
Feb 16, 2021

Label the diagram as follows, and let the sides of the square be s s and the radius of the blue circle be r r .

By the properties of a square, E G = E B = E C = E H = s 2 EG = EB = EC = EH = \cfrac{s}{2} , G A = 2 s 2 GA = \cfrac{\sqrt{2}s}{2} , and E G F = 45 ° \angle EGF = 45° .

Then E F = H F H E = r s 2 EF = HF - HE = r - \cfrac{s}{2} and F G = F A G A = r 2 s 2 FG = FA - GA = r - \cfrac{\sqrt{2}s}{2} .

By the law of cosines on E F G \triangle EFG , E F 2 = E G 2 + F G 2 2 E G F G cos E G F EF^2 = EG^2 + FG^2 - 2 \cdot EG \cdot FG \cdot \cos \angle EGF , or ( r s 2 ) 2 = ( s 2 ) 2 + ( r 2 s 2 ) 2 2 ( s 2 ) ( r 2 s 2 ) cos 45 ° \bigg(r - \cfrac{s}{2}\bigg)^2 = \bigg(\cfrac{s}{2} \bigg)^2 + \bigg(r - \cfrac{\sqrt{2}s}{2} \bigg)^2 - 2 \cdot \bigg(\cfrac{s}{2} \bigg) \cdot \bigg(r - \cfrac{\sqrt{2}s}{2} \bigg) \cdot \cos 45° , which solves to r = 2 + 3 2 7 s r = \cfrac{2 + 3\sqrt{2}}{7}s .

Therefore, A c A s = π r 2 s 2 = π ( 2 + 3 2 7 s ) 2 s 2 = 2 ( 11 + 6 2 ) 49 π \cfrac{A_c}{A_s} = \cfrac{\pi r^2}{s^2} = \cfrac{\pi (\frac{2 + 3\sqrt{2}}{7}s)^2}{s^2} = \cfrac{2(11 + 6\sqrt{2})}{49}\pi , so that α = 2 \alpha = 2 , β = 11 \beta = 11 , γ = 6 \gamma = 6 , ω = 49 \omega = 49 , and α + β + γ + ω = 68 \alpha + \beta + \gamma + \omega = \boxed{68} .

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