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In the above diagram square A B C D ABCD is tangent to the circle at point P P and vertices C C and D D are on the circle.

Let A A be the area of the red regions. If λ \lambda and ω \omega are the smallest coprime positive integers such that A A A B C D = α arcsin ( λ ω ) β p \dfrac{A}{A_{ABCD}} = \dfrac{\alpha\arcsin \left(\frac{\lambda}{\omega}\right) - \beta}{p} , where α \alpha , β \beta , λ \lambda , ω \omega , and p p are coprime positive integers, find α + β + λ + ω + p \alpha + \beta + \lambda + \omega + p .


The answer is 69.

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1 solution

Rocco Dalto
Dec 24, 2020

Let a a be a side of square A B C D ABCD .

C O D \triangle{COD} is an isosceles triangle O Q \implies \overline{OQ} is the perpendicular bisector of base C D \overline{CD} \implies

C Q = Q D = a 2 \overline{CQ} = \overline{QD} = \dfrac{a}{2}

In right C O Q \triangle{COQ} we have r 2 = a 2 2 a r + r 2 + a 2 4 a ( 5 a 8 r ) = 0 r^2 = a^2 - 2ar + r^2 + \dfrac{a^2}{4} \implies a(5a - 8r) = 0

and a 0 r = 5 a 8 a \neq 0 \implies r = \dfrac{5a}{8}

B M = a 2 ( a r ) = 2 r a = a 4 \overline{BM} = a - 2(a - r) = 2r - a = \dfrac{a}{4} and sin ( θ ) = a 2 r = 4 5 \sin(\theta) = \dfrac{a}{2r} = \dfrac{4}{5} \implies

Area of sector M O P MOP is A s = 1 2 θ r 2 = 1 2 arcsin ( 4 5 ) ( 25 64 ) a 2 = A_{s} = \dfrac{1}{2}\theta r^2 = \dfrac{1}{2}\arcsin(\dfrac{4}{5})(\dfrac{25}{64})a^2 = 25 128 arcsin ( 4 5 ) a 2 \dfrac{25}{128}\arcsin(\dfrac{4}{5})a^2

and

Area of trapezoid P B M O PBMO is A P B M O = 1 2 ( r + B M ) ( a 2 ) = 1 2 ( 5 a 8 + a 4 ) ( a 2 ) = 7 32 a 2 A_{PBMO} = \dfrac{1}{2}(r + \overline{BM})(\dfrac{a}{2}) = \dfrac{1}{2}(\dfrac{5a}{8} + \dfrac{a}{4})(\dfrac{a}{2}) = \dfrac{7}{32}a^2

A g r e e n = 2 ( A P B M O A s ) = 2 ( 7 32 25 128 arcsin ( 4 5 ) ) a 2 = \implies A_{green} = 2(A_{PBMO} - A_{s}) = 2(\dfrac{7}{32} - \dfrac{25}{128}\arcsin(\dfrac{4}{5}))a^2 =

( 28 25 arcsin ( 4 5 ) 64 ) a 2 (\dfrac{28 - 25\arcsin(\dfrac{4}{5})}{64})a^2

In M O C \triangle{MOC} we have sin ( θ ) = a r r = 3 5 θ = arcsin ( 3 5 ) \sin(\theta^{*}) = \dfrac{a - r}{r} = \dfrac{3}{5} \implies \theta^{*} = \arcsin(\dfrac{3}{5}) \implies

2 θ = 2 arcsin ( 3 5 ) 2\theta^{*} = 2\arcsin(\dfrac{3}{5}) \implies area of sector M O C MOC is A s = 1 2 ( 2 θ ) r 2 = 25 64 arcsin ( 3 4 ) a 2 A_{s^{*}} = \dfrac{1}{2}(2\theta)r^2 = \dfrac{25}{64}\arcsin(\dfrac{3}{4})a^2

and A M O C = 1 2 ( 2 ( a r ) ) ( a 2 ) = ( 3 a 8 ) ( a 2 ) = 3 16 a 2 A_{\triangle{MOC}} = \dfrac{1}{2}(2(a - r))(\dfrac{a}{2}) = (\dfrac{3a}{8})(\dfrac{a}{2}) = \dfrac{3}{16}a^2

A b l u e = 2 ( A s A M O C ) = 50 arcsin ( 3 5 ) 24 64 a 2 \implies A_{blue} = 2(A_{s^{*}} - A_{\triangle{MOC}}) = \dfrac{50\arcsin(\dfrac{3}{5}) - 24}{64}a^2

From above A s = 25 128 arcsin ( 4 5 ) a 2 A_{s} = \dfrac{25}{128}\arcsin(\dfrac{4}{5})a^2 and A C O R = 1 2 ( a r ) ( a 2 ) = 1 2 ( 3 a 8 ) ( a 2 ) = A_{\triangle{COR}} = \dfrac{1}{2}(a - r)(\dfrac{a}{2}) = \dfrac{1}{2}(\dfrac{3a}{8})(\dfrac{a}{2}) =

3 32 a 2 A p u r p l e = 2 ( A s A C O R ) = 25 arcsin ( 4 5 ) 12 64 a 2 \dfrac{3}{32}a^2 \implies A_{purple} = 2(A_{s} - A_{\triangle{COR}}) = \dfrac{25\arcsin(\dfrac{4}{5}) - 12}{64}a^2 \implies

A A A B C D = A g r e e n + A b l u e + A p u r p l e A A B C D = 25 arcsin ( 3 5 ) 4 32 = \dfrac{A}{A_{ABCD}} = \dfrac{A_{green} + A_{blue} + A_{purple}}{A_{ABCD}} = \dfrac{25\arcsin(\dfrac{3}{5}) - 4}{32} = α arcsin ( λ ω ) β p \dfrac{\alpha\arcsin \left(\frac{\lambda}{\omega}\right) - \beta}{p} \implies

α + β + λ + ω + p = 69 \alpha + \beta + \lambda + \omega + p = \boxed{69} .

Below is an alternative approach: \textbf{Below is an alternative approach:}

From above A s = 25 128 arcsin ( 4 5 ) a 2 2 A s = 25 64 arcsin ( 4 5 ) a 2 A_{s} = \dfrac{25}{128}\arcsin(\dfrac{4}{5})a^2 \implies 2A_{s} = \dfrac{25}{64}\arcsin(\dfrac{4}{5})a^2

and A M O N = 1 2 ( a ) ( a r ) = 1 2 ( 3 8 ) a 2 = 3 16 a 2 A_{\triangle{MON}} = \dfrac{1}{2}(a)(a - r) = \dfrac{1}{2}(\dfrac{3}{8})a^2 = \dfrac{3}{16}a^2

\implies Area of segment is M P N MPN is A s = 2 A s A M O N = 25 arcsin ( 4 5 ) 12 64 a 2 A_{s^{*}} = 2A_{s} - A_{\triangle{MON}} = \dfrac{25\arcsin(\dfrac{4}{5}) - 12}{64}a^2

A w h i t e = A M C N D + A s = 2 ( 3 8 ) a 2 + A s = A_{white} = A_{MCND} + A_{s^{*}} = 2(\dfrac{3}{8})a^2 + A_{s^{*}} = ( 3 4 + 25 arcsin ( 4 5 ) 12 64 ) a 2 = (\dfrac{3}{4} + \dfrac{25\arcsin(\dfrac{4}{5}) - 12}{64})a^2 =

36 + 25 arcsin ( 4 5 ) 64 a 2 A T = 25 64 π a 2 A w h i t e = \dfrac{36 + 25\arcsin(\dfrac{4}{5})}{64}a^2 \implies A_{T} = \dfrac{25}{64}\pi a^2 - A_{white} = 25 π 25 arcsin ( 4 5 ) 36 64 a 2 \dfrac{25\pi - 25\arcsin(\dfrac{4}{5}) - 36}{64}a^2

Form above A g r e e n = 28 25 arcsin ( 4 5 ) 64 a 2 A_{green} = \dfrac{28 - 25\arcsin(\dfrac{4}{5})}{64}a^2 \implies

A = A T + A g r e e n = 25 π 50 arcsin ( 4 5 ) 8 64 a 2 A = A_{T} + A_{green} = \dfrac{25\pi - 50\arcsin(\dfrac{4}{5}) - 8}{64}a^2

Letting θ = arcsin ( 3 5 ) A = 25 π 50 ( π 2 θ ) 8 64 a 2 = \theta^{**} = \arcsin(\dfrac{3}{5}) \implies A = \dfrac{25\pi - 50(\dfrac{\pi}{2} - \theta^{**}) - 8}{64}a^2 =

25 arcsin ( 3 5 ) 4 32 a 2 A A A B C D = \dfrac{25\arcsin(\dfrac{3}{5}) - 4}{32}a^2 \implies \dfrac{A}{A_{ABCD}} = 25 arcsin ( 3 5 ) 4 32 = α arcsin ( λ ω ) β p \dfrac{25\arcsin(\dfrac{3}{5}) - 4}{32} = \dfrac{\alpha\arcsin \left(\frac{\lambda}{\omega}\right) - \beta}{p}

α + β + λ + ω + p = 69 \implies \alpha + \beta + \lambda + \omega + p = \boxed{69} .

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