Inscribed Circle!

Geometry Level pending

The pink parabola x 2 + 2 3 x y + 3 y 2 + 2 3 x 2 y = 0 x^2 + 2\sqrt{3}xy + 3y^2 + 2\sqrt{3}x - 2y = 0 is tangent to a circle with radius 1 1 at points P P and Q Q as shown above and the green parabola 2 x 2 4 3 x y 6 y 2 + 2 3 x 2 y + 9 = 0 -2x^2 - 4\sqrt{3}xy - 6y^2 + 2\sqrt{3}x - 2y + 9 = 0 intersects the circle at points A , A, P P and Q Q .

Find the sum of the areas of regions R 1 , R 2 R_{1}, R_{2} and R 3 R_{3} .


The answer is 1.04719755.

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1 solution

Rocco Dalto
Apr 28, 2020

Using the rotation equations:

x = x cos ( θ ) y sin ( θ ) x = x'\cos(\theta) - y'\sin(\theta)

y = x sin ( θ ) + y cos ( θ ) y = x'\sin(\theta) + y'\cos(\theta)

For x 2 + 2 3 x y + 3 y 2 + 2 3 x 2 y = 0 x^2 + 2\sqrt{3}xy + 3y^2 + 2\sqrt{3}x - 2y = 0 we have:

x cos 2 ( θ ) x y sin ( 2 θ ) + y sin 2 ( θ ) + 3 ( x 2 sin 2 ( θ ) + x y sin ( 2 θ ) + y sin 2 ( θ ) ) + x'\cos^2(\theta) - x'y'\sin(2\theta) + y'\sin^2(\theta) + 3(x'^2\sin^2(\theta) + x'y'\sin(2\theta) + y'\sin^2(\theta)) +

2 3 ( x y cos ( 2 θ ) + ( x 2 y 2 ) sin ( θ ) cos ( θ ) ) + 2 3 ( x cos ( θ ) y sin ( θ ) ) 2\sqrt{3}(x'y'\cos(2\theta) + (x'^2 - y'^2)\sin(\theta)\cos(\theta)) + 2\sqrt{3}(x'\cos(\theta) - y'\sin(\theta))

2 ( x sin ( θ ) + y cos ( θ ) ) = 0 - 2(x'\sin(\theta) + y'\cos(\theta)) = 0

Setting x y x'y' term to zero we obtain:

2 sin ( 2 θ ) + 2 3 cos ( 2 θ ) = 0 tan ( 2 θ ) = 3 2 θ = 2 π 3 θ = π 3 2\sin(2\theta) + 2\sqrt{3}\cos(2\theta) = 0 \implies \tan(2\theta) = -\sqrt{3} \implies 2\theta = \dfrac{2\pi}{3} \implies \theta = \dfrac{\pi}{3}

Replacing θ = π 3 \theta = \dfrac{\pi}{3} in the above equation 4 x 2 4 y = 0 y = x 2 \implies 4x'^2 - 4y' = 0 \implies y' = x'^2

Similarly for 2 x 2 4 3 x y 6 y 2 + 2 3 x 2 y + 9 = 0 -2x^2 - 4\sqrt{3}xy - 6y^2 + 2\sqrt{3}x - 2y + 9 = 0 we obtain y = 2 x 2 + 9 4 y' = -2x'^2 + \dfrac{9}{4} .

Working from x y x'y' system and minimizing the distance O P OP we have:

D = d 2 = x 2 + ( x 2 y 0 ) 2 d D d x = x ( 2 x 2 ( 2 y 0 1 ) ) = 0 D = d^2 = x'^2 + (x'^2 - y'_{0})^2 \implies \dfrac{dD}{dx'} = x'(2x'^2 - (2y'_{0} - 1)) = 0 \implies

x = ± 2 y 0 1 2 2 y 0 1 2 + ( 2 y 0 1 2 y 0 ) 2 = 1 x' = \pm\sqrt{\dfrac{2y'_{0} - 1}{2}} \implies \dfrac{2y'_{0} - 1}{2} + (\dfrac{2y'_{0} - 1}{2} - y'_{0})^2 = 1

2 y 0 1 2 = 3 4 y 0 = 5 4 A : ( 0 , 9 4 ) \implies \dfrac{2y'_{0} - 1}{2} = \dfrac{3}{4} \implies y'_{0} = \dfrac{5}{4} \implies A:(0,\dfrac{9}{4}) and x = ± 3 2 x' = \pm\dfrac{\sqrt{3}}{2}

P : ( 3 2 , 0 ) \implies P:(\dfrac{\sqrt{3}}{2},0) and Q : ( 3 2 , 0 ) Q:(-\dfrac{\sqrt{3}}{2},0) and the equation of the circle in the x y x'y' system is

x 2 + ( y 5 4 ) 2 = 1 x'^2 + (y' - \dfrac{5}{4})^2 = 1

For R 1 R_{1} and R 2 : R_{2}:

I = 2 0 3 2 ( 2 x 2 + 9 4 ) ( 5 4 1 x 2 ) d x = I = 2\displaystyle\int_{0}^{\dfrac{\sqrt{3}}{2}} (-2x'^2 + \dfrac{9}{4}) - (\dfrac{5}{4} - \sqrt{1 - x'^2}) \:\ dx' =

2 0 3 2 ( 2 x 2 + 1 + 1 x 2 ) d x 2\displaystyle\int_{0}^{\dfrac{\sqrt{3}}{2}} (-2x'^2 + 1 + \sqrt{1 - x'^2}) \:\ dx' .

Letting x = sin ( θ ) d x = cos ( θ ) x' = \sin(\theta) \implies dx' = \cos(\theta) \implies

I = 2 ( ( 2 x 3 3 + x ) 0 3 2 + ( 1 2 ( θ + 1 2 sin ( 2 θ ) ) ) 0 π 3 ) = I = 2((-\dfrac{2x'^3}{3} + x')|_{0}^{\dfrac{\sqrt{3}}{2}} + (\dfrac{1}{2}(\theta + \dfrac{1}{2}\sin(2\theta)))|_{0}^{\dfrac{\pi}{3}}) =

2 ( 3 3 8 + π 6 ) = 3 3 4 + π 3 2(\dfrac{3\sqrt{3}}{8} + \dfrac{\pi}{6}) = \dfrac{3\sqrt{3}}{4} + \dfrac{\pi}{3}

A 1 = π I = 2 π 3 3 3 4 \implies \boxed{A_{1} = \pi - I = \dfrac{2\pi}{3} - \dfrac{3\sqrt{3}}{4}}

For R 3 : R_{3}:

For A 2 = 2 0 3 2 ( ( 5 4 1 x 2 ) x 2 ) d x = A_{2} = 2\displaystyle\int_{0}^{\dfrac{\sqrt{3}}{2}} ((\dfrac{5}{4} - \sqrt{1 - x'^2}) - x'^2) \:\ dx =

2 0 3 2 ( 5 4 x 2 1 x 2 ) d x 2\displaystyle\int_{0}^{\dfrac{\sqrt{3}}{2}} (\dfrac{5}{4} - x'^2 - \sqrt{1 - x'^2}) \:\ dx'

Letting x = sin ( θ ) d x = cos ( θ ) x' = \sin(\theta) \implies dx' = \cos(\theta) \implies

A 2 = 2 ( 5 4 x x 3 3 0 3 2 1 2 ( θ + 1 2 sin ( 2 θ ) ) 0 π 3 ) = A_{2} = 2(\dfrac{5}{4}x' - \dfrac{x'^3}{3}|_{0}^{\dfrac{\sqrt{3}}{2}} - \dfrac{1}{2}(\theta + \dfrac{1}{2}\sin(2\theta))|_{0}^{\dfrac{\pi}{3}}) =

2 ( 4 3 8 3 8 π 6 ) = 3 3 4 π 3 2(4\dfrac{\sqrt{3}}{8} - \dfrac{\sqrt{3}}{8} - \dfrac{\pi}{6}) = \dfrac{3\sqrt{3}}{4} - \dfrac{\pi}{3}

A 2 = 3 3 4 π 3 \boxed{A_{2} = \dfrac{3\sqrt{3}}{4} - \dfrac{\pi}{3}}

\implies the desired area A = A 1 + A 2 = π 3 1.04719755 A = A_{1} + A_{2} = \dfrac{\pi}{3} \approx \boxed{1.04719755}

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