Abnormal But Cute Polygon

Geometry Level 1

The figure to the right shows an irregular hexagon with six circles of radius 1, where the hexagon's vertices are the circles' centers.

Find the sum of the areas of the black regions.


The answer is 6.283185.

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5 solutions

Mena Sameh
Jan 30, 2016

what about converting the general case into a standard one :D

Then we have θ = 90 × 4 + 180 × 2 = 720 = 2 c i r c l e s \theta=90\times{4}+180\times{2} = 720 = 2 circles 2 × A r e a ( c i r c l e ) = 4 × π × r 2 = 2 π = 6.28 2\times{Area(circle)} = 4\times{π}\times{r^2} = \boxed{2π = 6.28}

That's nice, I love creativity +1

Ahmed Obaiedallah - 5 years, 4 months ago

Nice! how do we prove this?

Paola Ramírez - 5 years, 3 months ago

I am in love with this solution. +1

Akeel Howell - 4 years, 4 months ago
Paola Ramírez
Jan 28, 2016

As the circles have equal radius, the black sectors can be joined to make circles. A circle is completed by 360 ° 360° so firstable calculate the sum of angles of a hexagon as follows:

180 ° × 6 360 ° = 720 ° 180°\times 6- 360°=\boxed{720°}

With 720 ° 720° can be maked to circles so black area = 2 × π × 1 × 1 = 2 π =2\times \pi \times 1\times1=\boxed{2\pi}

I gave up because I typed in 2pi and couldn't figure out what to do. Maybe you could add "to the nearest millionth?"

Josiah Kiok - 5 years, 2 months ago

I wrote 6,28, it said "The answer must be a decimal number". Isn't 6,28 a decimal number?

Domenico Franceschelli - 5 years, 4 months ago

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no 6,28 isn't a decimal no. but 6.28 is :P

Baldev Rinwa - 5 years, 4 months ago

Using decimal, this would be 6.28 :)

Arshia Dar - 5 years, 4 months ago

@Domenico Franceschelli Did you wrote 6,28 or 6.28? If you wrote the first one, your answer was probably not accepted as a valid one.

Milan Milanic - 5 years, 4 months ago

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Then it was my fault: I wrote 6,28 instead of 6.28. In Italy we use commas as decimal marks :)

Domenico Franceschelli - 5 years, 4 months ago

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@Domenico Franceschelli I hear you loud and clear. In Serbia is the same. Luckily I am into programming, so I know that for type double or float dots are used instead of commas. :)

Ok to be more precise. No one knows what should be used here, but I usually use comas.

Milan Milanic - 5 years, 4 months ago

The sum of the interior angles is 72 0 720^\circ . One circle is 36 0 360^\circ . So we need to find the area of 720 360 = 2 \dfrac{720}{360}=2 circles. We have

A = 2 ( π ) ( 1 2 ) A=2(\pi)(1^2) \approx 6.283185307 \boxed{6.283185307}

This is how I did it.

A Former Brilliant Member - 1 year, 7 months ago
Finn C
Apr 10, 2016

I figured that the black area could fit in to 2 black circles. From there I just found the area of one circle (1 x 1 x 3.14) and multiplied it by 2. Therefore, the answer would be 3.14 x 2: 6.28

I assumed an arbitrary shape, took the top and bottom circles to infinity, and only the larger areas remain, all black, giving 2 pi.

Jim Bausch - 4 years, 10 months ago

The sum of all n-irregular angles can be determined by ( n 2 ) 180 (n - 2)180 . The sum of all angles of the hexagon is ( 6 2 ) 180 = 4 180 = 720 (6 - 2)180 = 4 \cdot 180 = 720 .

So, the shaded area is 720 360 1 2 π = 2 π 6.28 \frac { 720 }{ 360 } \cdot { 1 }^{ 2 }\pi =2\pi \approx 6.28 .

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