Who Ate My Pi?

Geometry Level 3

In the diagram, A B AB is the diameter of the small circle, B C BC and A C AC are radii of the big circle, and A B = B C = A C = 2 12 4 AB = BC = AC = 2\sqrt[4]{12} . Find the area of the blue region.


The answer is 9.

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1 solution

Chew-Seong Cheong
Sep 28, 2018

Let A B = B C = A C = 2 12 4 = 2 r AB=BC=AC=2\sqrt[4]{12}=2r . r = 12 4 r=\sqrt[4]{12} . We note that the top and down segments (in white) of the small circle are identical. The area of one of this small segment is

A 1 = Area of 6 0 sector Area of equilateral triangle = π r 2 6 1 2 r 2 sin 6 0 = π r 2 6 3 r 2 4 \begin{aligned} A_1 & = \text{Area of }60^\circ \text{ sector} - \text{Area of equilateral triangle} \\ & = \frac {\pi r^2}6 - \frac 12r^2 \sin 60^\circ \\ & = \frac {\pi r^2}6 - \frac {\sqrt 3 r^2}4 \end{aligned}

Since the big segment A B AB (in white) of the big circle has a radius of 2 r 2r , its area is \(A_2 = 2^2A_1 = 4A_1). And the area of the blue area is given by:

\(\begin{align} A_1 & = \text{Area of the }{\color{blue}\text{blue }} \text{circle} - 2A_1 - \color{blue}A_2 & \small \color{blue} \text{Since }A_2 = 4A_1 \\ & = \pi r^2 - 6A_1 \\ & = \pi r^2 - 6 \left(\frac {\pi r^2}6 - \frac {\sqrt 3 r^2}4\right) \\ & = \frac {3 \sqrt 3 r^2}2 \\ & = \frac {3\sqrt 3}2 \left(\sqrt[4]{12}\right)^2 \\ & = \frac {3\sqrt 3}2 \left(2\sqrt 3\right) \\ & = \boxed 9 \end{align} \)

Great solution!

David Vreken - 2 years, 8 months ago

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