In the diagram, is the diameter of the small circle, and are radii of the big circle, and . Find the area of the blue region.
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Let A B = B C = A C = 2 4 1 2 = 2 r . r = 4 1 2 . 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 = 6 π r 2 − 2 1 r 2 sin 6 0 ∘ = 6 π r 2 − 4 3 r 2
Since the big segment A B (in white) of the big circle has a radius of 2 r , its area is \(A_2 = 2^2A_1 = 4A_1). And the area of the blue area is given by:
\(\begin{align} A_1 & = \text{Area of the }{\color{blue}\text{blue }} \text{circle} - 2A_1 - \color{blue}A_2 & \small \color{blue} \text{Since }A_2 = 4A_1 \\ & = \pi r^2 - 6A_1 \\ & = \pi r^2 - 6 \left(\frac {\pi r^2}6 - \frac {\sqrt 3 r^2}4\right) \\ & = \frac {3 \sqrt 3 r^2}2 \\ & = \frac {3\sqrt 3}2 \left(\sqrt[4]{12}\right)^2 \\ & = \frac {3\sqrt 3}2 \left(2\sqrt 3\right) \\ & = \boxed 9 \end{align} \)