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In Right A B C , D C = 3 2 x , D E = x 2 \triangle{ABC}, \overline{DC} = \dfrac{\sqrt{3}}{2}x, \overline{DE} = \dfrac{x}{2} , A F \overline{AF} is perpendicular bisector of B D \overline{BD} and D P DP is an arc of a circle with center at C C .

Let A R 1 A_{R_{1}} and A R 2 A_{R_{2}} be the areas of the green and pink regions respectively.

If A R 1 A R 2 = ( α β ) ( α β π ) β β \dfrac{A_{R_{1}}}{A_{R_{2}}} = \dfrac{(\alpha - \sqrt{\beta})(\alpha\sqrt{\beta} - \pi)}{\beta - \sqrt{\beta}} , where α \alpha and β \beta are coprime positive integers, find α + β \alpha + \beta .


The answer is 5.

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2 solutions

David Vreken
Dec 21, 2020

Since D E = 1 2 x DE = \cfrac{1}{2}x and D C = 3 2 x DC = \cfrac{\sqrt{3}}{2}x , E C D = 30 ° \angle ECD = 30° .

R 1 R_1 is the difference between C D E \triangle CDE and sector C D P CDP , so A R 1 = 1 2 D C D E 30 ° 360 ° π D C 2 = 1 2 3 2 x 1 2 x 30 ° 360 ° π ( 3 2 x ) 2 = 1 16 ( 2 3 π ) x 2 A_{R_1} = \cfrac{1}{2} \cdot DC \cdot DE - \cfrac{30°}{360°} \pi DC^2 = \cfrac{1}{2} \cdot \cfrac{\sqrt{3}}{2}x \cdot \cfrac{1}{2}x - \cfrac{30°}{360°} \pi \bigg(\cfrac{\sqrt{3}}{2}x\bigg)^2 = \cfrac{1}{16}(2\sqrt{3} - \pi)x^2 .

Since A F AF is the perpendicular bisector of B D BD and A B A D AB \perp AD , A B = A D AB = AD .

Since A B C D E C \triangle ABC \sim \triangle DEC by AA similarity, D E D C = A B A C \cfrac{DE}{DC} = \cfrac{AB}{AC} . Letting a = A B = A C a = AB = AC , 1 2 x 3 2 x = a a + 3 2 x \cfrac{\frac{1}{2}x}{\frac{\sqrt{3}}{2}x} = \cfrac{a}{a + \frac{\sqrt{3}}{2}x} , which solves to a = 1 4 ( 3 + 3 ) x a = \cfrac{1}{4}(3 + \sqrt{3})x .

That means A R 2 = 1 2 D E A D = 1 2 1 2 x 1 4 ( 3 + 3 ) x = 1 16 ( 3 + 3 ) x 2 A_{R_2} = \cfrac{1}{2} \cdot DE \cdot AD = \cfrac{1}{2} \cdot \cfrac{1}{2}x \cdot \cfrac{1}{4}(3 + \sqrt{3})x = \cfrac{1}{16}(3 + \sqrt{3})x^2 .

So A R 1 A R 2 = 1 16 ( 2 3 π ) x 2 1 16 ( 3 + 3 ) x 2 = 2 3 π 3 + 3 = ( 3 3 ) 2 ( 2 3 π ) ( 3 3 ) 2 ( 3 + 3 ) = ( 2 3 ) ( 2 3 π ) 3 3 \cfrac{A_{R_1}}{A_{R_2}} = \cfrac{\frac{1}{16}(2\sqrt{3} - \pi)x^2}{\frac{1}{16}(3 + \sqrt{3})x^2} = \cfrac{2\sqrt{3} - \pi}{3 + \sqrt{3}} = \cfrac{(3 - \sqrt{3})^2(2\sqrt{3} - \pi)}{(3 - \sqrt{3})^2(3 + \sqrt{3})} = \cfrac{(2 - \sqrt{3})(2\sqrt{3} - \pi)}{3 - \sqrt{3}} .

Therefore, α = 2 \alpha = 2 , β = 3 \beta = 3 , and α + β = 5 \alpha + \beta = \boxed{5} .

Rocco Dalto
Dec 20, 2020

A \angle{A} is a right angle and A F \overline{AF} bisects B D A B D \overline{BD} \implies \triangle{ABD} is an isosceles right triangle

A D = a B D = 2 a \implies \overline{AD} = a \implies \overline{BD} = \sqrt{2}a and A B C \triangle{ABC} is a ( 30 , 60 , 90 ) (30,60,90) triangle \implies

A C = 3 2 x = 3 x + 2 a 2 = 3 a x = \overline{AC} = \dfrac{\sqrt{3}}{2}x = \dfrac{\sqrt{3}x + 2a}{2} = \sqrt{3}a \implies x = 2 ( 3 1 ) 3 a = 2 ( 3 3 ) 3 a \dfrac{2(\sqrt{3} - 1)}{\sqrt{3}}a = \dfrac{2(3 - \sqrt{3})}{3}a \implies

D E = x 2 = 3 3 3 a \overline{DE} = \dfrac{x}{2} = \dfrac{3 - \sqrt{3}}{3}a and h = E G = x 2 sin ( 4 5 ) = ( 3 3 3 ) ( 1 2 ) a h^{*} = \overline{EG} = \dfrac{x}{2}\sin(45^{\circ}) = (\dfrac{3 - \sqrt{3}}{3})(\dfrac{1}{\sqrt{2}})a \implies

A D B E = A R 2 = ( 1 2 ) ( 2 a ) ( 3 3 3 ) ( 1 2 ) a = A_{\triangle{DBE}} = A_{R_{2}} = (\dfrac{1}{2})(\sqrt{2a})(\dfrac{3 - \sqrt{3}}{3})(\dfrac{1}{\sqrt{2}})a = 3 3 6 a 2 \boxed{\dfrac{3 - \sqrt{3}}{6}a^2}

and

A D E C = 3 8 x 2 = 3 8 ( 4 9 ) ( 12 6 3 ) a 2 = A_{\triangle{DEC}} = \dfrac{\sqrt{3}}{8}x^2 = \dfrac{\sqrt{3}}{8}(\dfrac{4}{9})(12 - 6\sqrt{3})a^2 = 3 3 9 ( 2 3 ) a 2 = 3 3 ( 2 3 ) a 2 \dfrac{3\sqrt{3}}{9}(2 - \sqrt{3})a^2 = \dfrac{\sqrt{3}}{3}(2 - \sqrt{3})a^2

and

r = 3 2 x = ( 3 1 ) a r = \dfrac{\sqrt{3}}{2}x = (\sqrt{3} - 1)a \implies the area of the sector A 1 = ( 1 2 ) ( π 6 ) ( 3 1 ) 2 a 2 = A_{1} = (\dfrac{1}{2})(\dfrac{\pi}{6})(\sqrt{3} - 1)^2a^2 =

π 12 ( 4 2 3 ) a 2 = π 6 ( 2 3 ) a 2 \dfrac{\pi}{12}(4 - 2\sqrt{3})a^2 = \dfrac{\pi}{6}(2 - \sqrt{3})a^2

A R 1 = A D E C A 1 = ( 2 3 ) ( 2 3 π 6 ) a 2 \implies A_{R_{1}} = A_{\triangle{DEC}} - A_{1} = (2 - \sqrt{3})(\dfrac{2\sqrt{3} - \pi}{6})a^2 \implies

A R 1 A R 2 = ( 2 3 ) ( 2 3 π ) 3 3 = \dfrac{A_{R_{1}}}{A_{R_{2}}} = \dfrac{(2 - \sqrt{3})(2\sqrt{3} - \pi)}{3 - \sqrt{3}} = ( α β ) ( α β π ) β β \dfrac{(\alpha - \sqrt{\beta})(\alpha\sqrt{\beta} - \pi)}{\beta - \sqrt{\beta}}

α + β = 5 \implies \alpha + \beta = \boxed{5} .

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