A Weird Area

Geometry Level 3

In the diagram, regular hexagon A B C D E F ABCDEF has sides of length 2 2 . Using A A , C C and E E as centers, portions of circles with radius 1 1 are drawn outside the hexagon. Using B B , D D and F F as centers, portions of circles with radius 1 1 are drawn inside the hexagon. These six circular arcs join together to form a curve. Determine the area of the shaded region, both red and green, enclosed by this curve.


Problem and Image: courtesy waterloo university

6 3 + 3 π 6\sqrt{3}+3\pi 6 2 + π 6\sqrt{2}+\pi 2 3 + π 2\sqrt{3}+\pi 6 3 + 2 π 6\sqrt{3}+2\pi 6 3 + π 6\sqrt{3}+\pi

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2 solutions

let A A = area of the shaded region enclosed by the curve

A H A_H = area of the hexagon

A R A_R = area of the red region

A W A_W = area of the white region inside the hexagon

From the figure, we can see that A = A H A W + A R \boxed{A = A_H - A_W + A_R} .

Solving for the area of the hexagon

A H = A_H = 6 ( a r e a 6(area o f of o n e one e q u i l a t e r a l equilateral t r i a n g l e ) triangle)

A H = 6 ( 0.5 ) ( 2 2 ) ( 3 2 ) = 6 3 A_H = 6(0.5)(2^2)(\frac{\sqrt{3}}{2}) = 6\sqrt{3}

Solving for the area of the white region inside the hexagon

A W = 3 ( 120 360 ) ( π ) ( 1 2 ) = π A_W = 3(\frac{120}{360})(π)(1^2) = π

Solving for the area of the red region

A R = 3 ( 240 360 ) ( π ) ( 1 2 ) = 2 π A_R = 3(\frac{240}{360})(π)(1^2) = 2π

Solving for the area of the shaded region enclosed by the curve

A = A H A W + A R A = A_H - A_W + A_R

A = 6 3 π + 2 π = A = 6\sqrt{3} - π + 2π = 6 3 + π \boxed{6\sqrt{3} + π}

Yes, true. Thank you for posting a nice solution.

Hana Wehbi - 4 years, 2 months ago

Red each covers 240 degree sector of circle r=1.
White each eats awy 120 degree sector of circle r=1.
So surplus each 120 degrees over Hexagon area.
There are 3 such sectors to give 3 120==360 degrees, that is a circle area π 1 2 = π \pi*1^2 =\pi .
Total colored area =Hexagon area+surplus area=6
equilateral triangles sides = 2 +surplus area.
Total colored area= 6 3 4 2 2 + π = 6 3 + π . 6*\dfrac{\sqrt3} 4*2^2+\pi=\Large \color{#D61F06}{6\sqrt3+\pi}.


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