Find the area of the shaded region in square centimeters

Geometry Level pending

Three identical circles which are externally tangent to each other are inscribed in an equilateral triangle as shown. Given that the the radius of each circle is 3 cm 3~\text{cm} , what is the area of the shaded region in cm 2 \text{cm}^2 rounded to the nearest integer?

38 cm 2 38~\text{cm}^2 35 cm 2 35~\text{cm}^2 32 cm 2 32~\text{cm}^2 26 cm 2 26~\text{cm}^2 29 cm 2 29~\text{cm}^2

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1 solution

Relevant wiki: Area of Triangles - Problem Solving - Easy

Consider my figure.

tan 30 = 3 x \tan 30 = \dfrac{3}{x} \implies x = 3 tan 30 x=\dfrac{3}{\tan 30}

It follows that A C = 2 ( 3 tan 30 ) + 6 = 6 tan 30 + 6 AC=2\left(\dfrac{3}{\tan 30}\right)+6=\dfrac{6}{\tan 30}+6 .

The area of an equilateral triangle is given by A = 3 4 a 2 A=\dfrac{\sqrt{3}}{4}a^2 where a a is the side length. The area of a circle is given by A = π r 2 A=\pi r^2 where r r is the radius.

From the figure, the area of the shaded region is equal to the area of the equilateral triangle minus the area of the three circles.

Thus,

A s h a d e d = 3 4 ( 6 tan 30 + 6 ) 2 3 ( π ) ( 3 2 ) 32 cm 2 A_{shaded}=\dfrac{\sqrt{3}}{4}\left(\dfrac{6}{\tan 30}+6\right)^2-3(\pi)(3^2) \approx \boxed{32~\text{cm}^2} .

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