Packed in the middle

Geometry Level 3

The area of the blue circular sector is exactly half that of the equilateral red trangle A B C \triangle{ABC} .

How much larger is A B C \triangle{ABC} compared to the green disc inscribed in the circular sector ?


The answer is 3.

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1 solution

Romain Bouchard
Jun 26, 2018

Let r r be the radius of the green inner circle and d d the distance between A A and the inner circle's center.

Let A b l u e A_{blue} be the area of the circular sector, A r e d A_{red} the area of the triangle A B C \triangle {ABC} and A g r e e n A_{green} the area of the inner circle.

We can write sin ( C A B ) = sin ( π 6 ) = r d d = 2 r \sin(\angle{CAB}) = \sin(\frac{\pi}{6}) = \frac{r}{d} \Rightarrow d = 2r .

Since A b l u e = π ( d + r ) 2 6 = A r e d 2 A_{blue} = \frac{\pi (d+r)^2}{6} = \frac{A_{red}}{2} , then A r e d = π ( d + r ) 2 3 = π ( 3 r ) 2 3 = 3 π r 2 = 3 A g r e e n A r e d A g r e e n = 3 A_{red} = \frac{\pi (d+r)^2}{3} = \frac{\pi (3r)^2}{3} = 3\pi r^2 = 3*A_{green} \Rightarrow \frac{A_{red}}{A_{green}}= \boxed{3}

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