Circles Abound

Geometry Level 1

Within the orange square, there are 4 circles each with radius 1. What is the area of the green region in the center?


Hint: Connect the centers of the circles.

2 π 6 2 \pi - 6 1 1 π 4 \frac{ \pi } { 4} 4 π 4 - \pi

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13 solutions

Chew-Seong Cheong
Mar 20, 2017

From the figure above, it is clear that the area of the green region is the area of the 2 × 2 2 \times 2 square minus the area of 4 quadrants of radius 1 or a circle of radius 1, or A = 2 × 2 π × 1 2 = 4 π A = 2 \times 2 - \pi \times 1^2 = \boxed{4-\pi} .

Nice solution! Diagram says it all

Agnishom Chattopadhyay - 4 years, 2 months ago

What application do you use to make the picture?

Fidel Simanjuntak - 4 years, 2 months ago

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Copy the image from the problem and then edit it in Paint.exe.

Chew-Seong Cheong - 4 years, 2 months ago

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Oohh.. I am still finding a good application to make a diagram.. Hehehe

Fidel Simanjuntak - 4 years, 2 months ago

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@Fidel Simanjuntak @Chung Kevin what application did you use to make the diagram?

Fidel Simanjuntak - 4 years, 2 months ago

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@Fidel Simanjuntak I create my images using OmniGraffle on a Mac.

Brilliant will improve on the images for problems that are popular, so it doesn't really matter what application you use. They use Figma to create vector images, but I rather use something I'm comfortable with. Others have created images using Paint, or even Microsoft word.

Chung Kevin - 4 years, 2 months ago

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@Chung Kevin Thanks for the advice!

Fidel Simanjuntak - 4 years, 2 months ago

Why would it be a square? I mean, why would all angles be 90 degree?

Tanmay Sahoo - 4 years, 2 months ago

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You are right. I have made an assumption.

Chew-Seong Cheong - 4 years, 2 months ago

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That's right! Thanks for pointing out that there isn't enough information to solve this problem completely (other than eyeballing the image).

I've now added a condition to ensure that the configuration is a square. Previously, the image had a unwritten assumption about the arrangement of the circles and their tangencies.

Calvin Lin Staff - 4 years, 2 months ago
Ojasee Duble
Mar 10, 2017

We get a square of side 2 units by joining all the centres of the circle.

The square has yellow part & green part.

The yellow part consists of 4 congruent quadrants of a circle. (They are congruent because they all have central angle 90 and radius 1)

Hence, the area of yellow part = Area of circle with radius 1 = π * 1 * 1 = π

Area of the square = side * side = 4

Therefore, Green region = Area of square - Area of yellow region = 4 - π

Why would it be a square? I mean, why would all angles be 90 degree?

Tanmay Sahoo - 4 years, 2 months ago

you did it the same as me. im a math prodidgy. i totaly agree with you

Nuwelin Dagne - 4 years, 2 months ago
Toshit Jain
Mar 9, 2017

If we join the centres of all circles , we will get a square with side = 2×radius of circle = 2 | Area of square = Area of four quadrants + Green Area | Since all circles are congruent , the four quadrants will make up a complete circle with radius = 1 | 2×2 = π×1×1 + Green Area | Therefore , Green Area = 4-π . ||

That's nice. Gives us the green region directly.

Chung Kevin - 4 years, 3 months ago

What I did is different

I put a square aroud it all, so the side of the square would be 2 * D = 4 * R = 4

So the square area is 4*4

now, the area of each circle is π, so I remove it from the square -> A = 4*4 - 4π = 4 ( 4 - π)

Now, the blank spaces and the green spaces sum up to 4(4-π)

The blank spaces on the sides sum up to to green area, also, the area of the top and bottom sum up to the same green area, while the corners sum up to the green area, so we have A = 4 *G

So 4G = 4(4-π) => G = 4-π

Fernando Favini - 4 years, 2 months ago

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Yes, this is an interesting approach.

Agnishom Chattopadhyay - 4 years, 2 months ago
Siva Budaraju
Mar 14, 2017

From this image we will only focus on what is inside the 2x2 square. We can see that the areas of the 4 quarter circles are each 1/4 π, so the total of those areas is π. The area of the square is 4, so the area of the green region is 4-π .

This is a nice way to look at the figure! We can picture the green area as four quarter circles removed from a square.

Pranshu Gaba - 4 years, 2 months ago

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exactly, this is how I did it

Siva Budaraju - 4 years, 2 months ago
Dan Ley
Mar 20, 2017

Focusing on one quadrant: Green Area = Square of sidelength 2 circle of radius 1 4 \text{Green Area}=\frac{\text{Square of sidelength } 2-\text{circle of radius }1}{4}

If we consider the entire diagram, the total green area is 4 times this amount, and is therefore a Square of sidelength 2 circle of radius 1 = 2 2 π = 4 π \text{Square of sidelength }2-\text{circle of radius }1=2^2-\pi=\boxed{4-\pi} .

This is probably the neatest solution I've seen here~~! Nicee

Pi Han Goh - 4 years, 2 months ago

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Thank you!!:)

Dan Ley - 4 years, 2 months ago
Venkatachalam J
Mar 19, 2017

Relevant wiki: Length and Area Problem Solving

Clearly from the diagram,

Area of Green Region=Area of Red Square - Area of the Circle Inside the Red Square= 2 2 ( π × 1 × 1 ) = 4 π 2^2-(π \times 1 \times 1 )= 4 - π

oh! I didn't thought about rearranging the green regions. The picture on the right makes it so much easier to solve the question. Thanks!

Pi Han Goh - 4 years, 2 months ago

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Thank you for the comments. Visualization & little imagination will give the solution in the fewest steps.

Venkatachalam J - 4 years, 2 months ago
Aaryan Maheshwari
Mar 10, 2017

As each circle has radius 1, area of all the 4 circles= π r 2 4 \pi r^2*4 = 4 π 4\pi . If we make a square on the boundary of the figure, we can notice that 1 1 more the same shape as the green region can be contained in the square. Thus, area of green region= 16 4 π 2 \frac{16-4\pi}{2} , which can be reduced to 4 π 4-\pi . That is the answer!

Actually, it is "total of 4 green regions in the square".

Your division is off. Instead, it should be 16 4 π 4 = 4 π \frac{ 16 - 4 \pi } { 4} = 4 - \pi .

Chung Kevin - 4 years, 3 months ago

Ok! Thanks for pointing out. I will check if I write a solution next time.

Aaryan Maheshwari - 4 years, 3 months ago

The 'white' area inside the square is 2 times the 'green' area.

Mansoor Ali - 4 years, 2 months ago

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Hmmm, what white area are you referring to?

Pi Han Goh - 4 years, 2 months ago

Area of green region = area of square - area of circle = 2 2 π ( 1 2 ) = 4 π 2^2 - π(1^2) = 4 - π

Good image. It makes your solution clear and easy to understand.

Christopher Boo - 4 years, 2 months ago
Shivang Bhawsar
Mar 13, 2017

Make a square of 4 by 4 units around the four circles, let the required green area be =x
So now, area of square - area of circle=4x
As we can see from symmetricity that the part between any two circle and the edge of the square has an area x/2 and there are 4 such parts so these parts have area 2x and the part between a circle and the corner of the square has an area x/4 and there are 4 such parts so they give an area x,
Now, area of square - area of circle = 16-4π
=x(green part) + 2x +x = 4x
So, x= (16-4π)/4= 4-π
Which is the required area.





Oh this is an interesting way to look at the figure. There will be a total of four times the green area in the big square.

Pranshu Gaba - 4 years, 2 months ago
Md,shafiqul Alom
May 19, 2017

Area=(2)^2-4*(pi\4)

Jay Jina
Mar 23, 2017

The green area is equivalent to the bits of area between the unit circle and the square subscribing it.

Easy.

Roy Bertoldo
Mar 23, 2017

Area of each circle = 1*∏ = ∏

Circumscribe one of the circles with a square with side = 2

Area of the square = 2*2=4

Area of that part of the square outside the circle = 4-∏

Area of the green area inside the square = (4-∏)/4

True for all four circles

Therefore the combined green area = 4*(4-∏)/4 = 4-∏

I assume the weirdly rendered character is pi. Your solution is very easy to follow!

Christopher Boo - 4 years, 2 months ago
Saurabh Chavan
Mar 20, 2017

There is four circle having same radius so we can have area of square by joining there circle is=4,and every circle contribute his 1/4th part so area of one circle is=3.14/4 (2)^2,and area covered by four circle in square is 4 (area of one circle covered) is directly=3.14 so we can write the soution as=4-3.14.hence solved.

Yes, that is a valid approach.

Agnishom Chattopadhyay - 4 years, 2 months ago

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