Squares and Circles!

Geometry Level 4

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 R A_{R} be the area of the red regions. If A R A A B C D = α β arcsin ( λ ω ) p \dfrac{A_{R}}{A_{ABCD}} = \dfrac{\alpha - \beta\arcsin \left(\frac{\lambda}{\omega}\right)}{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 126.

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2 solutions

Chew-Seong Cheong
Dec 22, 2020

Let the side length of square A B C D ABCD be 1 1 Then the area of square A B C D ABCD , [ A B C D ] = 1 [ABCD] = 1 . Let B E = x BE=x . By tangent-secant theorem , we have B E B C = B P 2 x 1 = ( 1 2 ) 2 x = 1 4 BE\cdot BC = BP^2 \implies x \cdot 1 = \left(\dfrac 12\right)^2 \implies x = \dfrac 14

Due to symmetry, we note that B E = P G = H Q = 1 4 BE=PG=HQ = \dfrac 14 . Since the diameter of the circle P Q = P H + H Q = 1 + 1 4 = 5 4 PQ = PH + HQ = 1 + \dfrac 14 = \dfrac 54 , the radius of the circle r = 5 8 r = \dfrac 58 .

Then the area of the red regions

A R = [ A B E F ] r e c t a n g l e [ E P F G ] s e g m e n t = [ A B E F ] r e c t a n g l e ( [ E P F O ] s e c t o r [ E F O ] t r i a n g l e ) = A F A B r 2 sin 1 ( E G O E ) + 1 2 E F O G = 1 1 4 5 2 8 2 sin 1 ( 1 2 8 5 ) + 1 2 1 ( 5 8 1 4 ) = 1 4 25 64 sin 1 ( 4 5 ) + 3 16 = 28 25 sin 1 ( 4 5 ) 64 \begin{aligned} A_R & = \overbrace{[ABEF]}^{\rm rectangle} - \overbrace{[EPFG]}^{\rm segment} \\ & = \overbrace{[ABEF]}^{\rm rectangle} - \bigg(\overbrace{[EPFO]}^{\rm sector} - \overbrace{[EFO]}^{\rm triangle} \bigg) \\ & = AF \cdot AB - r^2 \sin^{-1} \left(\frac {EG}{OE} \right) + \frac 12 \cdot EF \cdot OG \\ & = 1 \cdot \frac 14 - \frac {5^2}{8^2} \sin^{-1} \left( \frac 12 \cdot \frac 85 \right) + \frac 12 \cdot 1 \cdot \left(\frac 58 - \frac 14 \right) \\ & = \frac 14 - \frac {25}{64} \sin^{-1} \left(\frac 45 \right) + \frac 3{16} \\ & = \frac {28 - 25\sin^{-1} \left(\frac 45 \right)}{64} \end{aligned}

Since [ A B C D ] = 1 [ABCD]=1 , A R [ A B C D ] = A R \dfrac {A_R}{[ABCD]} = A_R and the required answer is 28 + 25 + 4 + 5 + 64 = 126 28+25+4+5+64 = \boxed{126} .

Nice one! When you use the tangent-secant theorem (first line), I believe it should be B E B C BE \cdot BC , not B E B D BE \cdot BD .

David Vreken - 5 months, 3 weeks ago

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Thanks, David, I have fixed it.

Chew-Seong Cheong - 5 months, 3 weeks ago
Rocco Dalto
Dec 21, 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 R = 2 ( A P B M O A s ) = 2 ( 7 32 25 128 arcsin ( 4 5 ) ) a 2 = \implies A_{R} = 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 A R A A B C D = (\dfrac{28 - 25\arcsin(\dfrac{4}{5})}{64})a^2 \implies \dfrac{A_{R}}{A_{ABCD}} = 28 25 arcsin ( 4 5 ) 64 = α β arcsin ( λ ω ) p \dfrac{28 - 25\arcsin(\dfrac{4}{5})}{64} = \dfrac{\alpha - \beta\arcsin(\dfrac{\lambda}{\omega})}{p}

α + β + λ + ω + p = 126 \implies \alpha + \beta + \lambda + \omega + p = \boxed{126} .

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