Blue Area vs. Yellow Area

Geometry Level 1

In the above figure, which shaded area is larger?

The small circles all have the same size.

Yellow region Equal area Blue region

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30 solutions

Timothy White
Mar 13, 2015

The ratio of a circle to the square that envelops it is the same regardless of the size. Multiplying it by 16 doesn't change the ratio, and as long as the squares are the same size and the circles all fit exactly, then the ratio is the same.

Nice observation! That's how I was thinking of this problem at first.

Chung Kevin - 6 years, 3 months ago

The way I got the answer was to think of the square as 8x8 so the big circle would have a diameter of 8, a radius of 4 and an area of 16pi. The smaller circles would have a diameter of 2, a radius of 1 and an area of pi each,then multiply that area of pi by the # of small circles and you get 16pi, which is the same as the big circle so therefore, the regions have the same area...... And when I was going to answer it, I didn't look at what I was clicking and unwittingly chose the yellow region.

Guy Alves - 6 years ago

Yes. I also scaled it down to 1/16. Ratio is the same.

chris clement - 5 years, 10 months ago

No proof is offered that ratio is the same, which only intuitively appears to be true.

Mason Kelsey - 3 years, 3 months ago

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Intuition is all you need when it comes to most geometry questions. The proof is that the area of a square is uniquely defined by a single unit quadratically (length), and the circle is the same (radius). The radius of a circle and the length of a square scale at a linear rate when the other changes, so the ratio would stay the same.

Timothy White - 3 years, 1 month ago

All squares are similar. All circles are similar. Therefore all compound shapes made up of a square and the inscribed circle are similar. Or in other words, exactly what Tim said. There is nothing wrong with his proof.

Chris Maitland - 2 years, 7 months ago

Similar figures. Easy as that.

Anthony Cutler - 1 year, 11 months ago
Vaibhav Prasad
Mar 9, 2015

Let the side of the square be equal to x x

The radius of the big circle comes to be x 2 \frac{x}{2} and hence the white area comes to be

π ( x 2 ) 2 = π x 2 4 \pi { \left( \frac { x }{ 2 } \right) }^{ 2 }=\boxed {\pi \frac { { x }^{ 2 } }{ 4 }}

In the second square the diameter of each circle would be x 4 \frac{x}{4} and hence the radius would be x 8 \frac{x}{8} . Thus the area of the smaller circles would be

π ( x 8 ) 2 = π x 2 64 \pi { \left( \frac { x }{ 8 } \right) }^{ 2 }=\quad \pi \frac { { x }^{ 2 } }{ 64 } . Since there are 16 16 circles, the total white area would be

π x 2 64 × 16 = π x 2 4 \pi \frac { { x }^{ 2 } }{ 64 } \times 16\quad =\quad \boxed {\pi \frac { { x }^{ 2 } }{ 4 } } .

Since the white area in both the squares is the same, the shaded areas would also be same (obviously :P)

How do you know that the squares have equal sides which has equal area? In the question, it is not mentioned.

Mishal Sheik - 5 years, 6 months ago

Was it surprising that the circular areas were the same? Is there some (deeper) reason why they should be equal?

Chung Kevin - 6 years, 3 months ago

I simply saw something about them being the same size, so they'd both have the same (or equal) shaded areas. Is it really as complicating as OP's post? I mean, I have no idea what he's talking about, but did I do it right? lol.

Matt Fay - 6 years, 3 months ago

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There are many different approaches to this problem. The one listed in this solution simply finds the white areas involved, show that they are equal, and thus shaded areas are equal.

The one given in several other solutions, show that the shaded area : total area is a constant in both figures, and hence the shaded area must be the same.

Chung Kevin - 6 years, 3 months ago

You are assuming that thr squares are the same size

A Bulgarino - 6 years, 2 months ago

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It's not given that there sizes are different and so you would have to go by the picture

Vaibhav Prasad - 6 years, 2 months ago

and going by the picture, since x represents the side of the square, you can see that both squares have the same side lengths;

My only critique on this method, since there are two separate squares, avoid using the variable x to represent both squares; using separate vars, say x and y, you can easily set a condition that they are equal (with explanations if required) to avoid this particular objection.

Johnathon Sage - 5 years, 9 months ago

Nice solution!

Sean Matthew Tan - 2 years, 9 months ago

Excellent answer

Amna Mushtaq - 6 years, 2 months ago

VERY NICE BRO

waqas ahmed - 6 years, 2 months ago
Aloy Locsin
Mar 11, 2015

big circle radius = r small circle radius = 1/4 r

A(yellow)=(pi)(r^2)

A(blue)=(16)(pi)(r/4)^2 =(16)(pi)(r^2)/(16) =(pi)(r^2)

nicely done.

Chung Kevin - 6 years, 3 months ago

Very neat solution, like it.

Andy Davies - 6 years, 3 months ago

Nice. The only thing I can recommend to improve your solution is using LaTeX, but otherwise neat solution!

Sean Matthew Tan - 2 years, 9 months ago
Jitesh Mittal
Mar 13, 2015

If we divide the red shaded square in 16 congruent squares such that each square contains exactly one circle.

Now consider the blue shaded square and one of the 16 congruent squares. In the above considered squares,we can directly state, without any calculation, that the ratio of shaded area and unshaded area in both is equal because both are squares in which the circle touches the boundary of the respective square.

Likewise we have the ratio of shaded area and unshaded area equal in all the 17 squares(16 red shaded+1 blue shaded).

So we can have, by properties of ratio and proportion, that, ratio of shaded area and unshaded area equal in both the larger red square and blue square and hence equal shaded area area as total areas of squares are same.

Nice, that's how I thought of this.

Chung Kevin - 6 years, 3 months ago

My solution is exactly as yours. Took me 2 seconds. O.5 sec to think and 1.5 sec to key in answer.

Josiah Yip - 6 years, 3 months ago
Ethan Shaikh
Mar 13, 2015

Just went with the logic that the shaded area that surrounds each circle in the red square is a percentage of the shaded area that surrounds the big circle in the blue square.

Good logic! My thoughts exactly.

Chung Kevin - 6 years, 3 months ago
Hero Miles
Mar 12, 2015

Let the radius of the small circles be x x . Then the area of one small circle is π x 2 πx^2 . The area of 16 16 small circles is 16 π x 2 16πx^2 . The length of a side of the square is 8 x 8x The area of the square is 64 x 2 64x^2

Assume the radius of one small circle is 1 1 unit. Then the difference between the areas gives the area of the red region which is is 64 16 π 64 - 16π or 16 ( 4 π ) 16(4 -π)

The radius of the large circle is 4 x 4x The area of the large circle is 16 π x 2 16πx^2 At this point we know the area of the large circle is equal to the area of 16 16 small circles.

The difference between the area of square and the area of the large circle gives the area of the blue region which is also 64 16 π 64 - 16π or 16 ( 4 π ) 16(4 - π)

So the area of the red region is equal to the area of the blue region.

Philip Choi
May 27, 2015

Let's say the the big circle has the radius of 1 and area of Pi. Then the square is 4 cm^2. The small circles have the quarter of the radius of the big circle. Therefore, the area of a small circle is 1/16*Pi. Since there are 16 circles, the area of small circles is Pi. Therefore, the shaded areas are equal.

That's good.

Chung Kevin - 6 years ago
Callum Goodall
May 19, 2015

let's pretend we live in a world where Pi = 1, it is a ratio after all and we'll give the edges of the square a value: 8 meaning for the big square r^2Pi=16 and each small square r^2Pi = 1 you can see that there are 16 small squares and their areas are 1 so the total area is 16, thus, they are equal!

Hm, there might be issues with pretending that we live in a world where PI = 1. It is better to just leave PI in your calculations.

Chung Kevin - 6 years ago
Akshay Gupta
Jun 26, 2017

Let Side Length of both Squares be "8a"

Then Radius of Large Circle in left with Yellow Region will be = 4a

So, Area of Yellow Region A X 1 \ce{A1} = Area of Square - Area of Circle = ( 8a x 8a ) - ( π x 4 a X 2 \ce{4a^2} )

= (64 - 16π) a X 2 \ce{a^2}

Radius of every Small Circle in right with Blue Region = a

So, Area of Blue Region A X 2 \ce{A2} = Area of Square - ( 16 x Area of Circle ) = ( 8a x 8a ) - { 16 x ( π x a X 2 \ce{a^2} )}

= (64 - 16π) a X 2 \ce{a^2}

A X 1 \ce{A1} = A X 2 \ce{A2} i.e., Both have equal area

Betty BellaItalia
May 17, 2017

Avner Lim
Dec 14, 2020

Let the radius of 1 small circle be r. The radius of the large circle is 4r. Total area of 16 little circles: 16 × (πr²) = 16πr² Total area of 1 big circle: π(4r)² = 4² × (πr²) = 16πr² 16πr² = 16πr² Thus, they have the same area.

Alkis Piskas
Oct 21, 2018

Lets' assign one unit to the radii of the 16 small circles. 1) The side of the circumscribing square is 8, si its area is 64 .
2) The total area of the 16 circles is 16π . 3) The blue area is the area of the square minus the total area of the 16 circles, i.e. 64 - 16π . 4) The radius of circumscribed circle at the letft is 4 (half of that ot the square), so its area is π4^2 = 16π . 5) The yellow area is the area of the square minus the area of the circumscribed circle, i.e. 64 - 16π , which is also the yellow area.

Angel Leon
Oct 5, 2018

Let R be the radius of the big circle. Let r be the radius of a small circle.

R = 4r.

Area of Big Circle => Pi * R^2 => Pi * 16 * r^2.

Area of the 16 small circles => 16 * Pi * r^ 2.

Both are the same thing, both subtract the same area from the square.

Un Owen
Apr 16, 2018

Let the radius of the big circle be r. Then, the area of the big circle is πr². The square around the big circle has the dimensions 2r x 2r, for an area of 4r². Then the area of the square minus that of the circle is 4r²-πr²=r²(4-π). The smaller circles have a radius of r/4. So their area is πr²/16 each. Since there are 16 of them, their total area is 16 x πr²/16, of πr². The blue region has the area of (2r x 2r)- πr². 4r²-πr²=r²(4-π)--which is the same as the yellow region. Thus, the regions have equal areas ☺☺☺☺

Stefan Yambolov
Mar 26, 2018

Yellow area, 4 shaded corners

Blue area, 64 shaded corners

The smaller circles have 1/4 the radius of he big one, so they each have 1/16th of it's area

1 yellow corner has the same area as 16 blue corners.

There are 4 yellow and 64 blue corners.

4*16=64

Same area.

Ahsim Nreiziev
Mar 25, 2018

Each square is the same size, so all we need to do is compare the white area in each square, to see which of the coloured areas is larger or if they are the same size.

If the radius of the big white circle in the left square is r , then said circle obviously has the familiar area of π × r 2 \pi \times r^2 .

Each of the small circles in the square on the right has radius of (1/4) r , which translates to an area of π × ( 1 4 r ) 2 = π × 1 16 r 2 \pi \times {(\frac{1}{4} r)}^2 = \pi \times \frac{1}{16} r^2 per small circle. There are 16 of them, so the total area of all the white circles in the blue square is 16 × π × 1 16 r 2 16 \times \pi \times \frac{1}{16} r^2 , which is just π × r 2 \pi \times r^2

In other words: the white areas in both squares have exactly the same size. Then what's left over must be the same size too. The answer to the question is thus E q u a l a r e a \large \color{#3D99F6}{\boxed{Equal \space area}} .

Ali Mahamamo
Aug 28, 2016

Left : 4 r2 - pi r2 right : 4 r2 - 16 pi(r/4)2

Ganesh Gajavelli
May 22, 2016

If the radius of the smaller circle is "r", then the area of that circle is pi (r^2). Thus, the addition of the 16 small circles of the right-side diagram is 16(pi (r^2)). The length of one side of the square that the circles are within is equal to the length of the diameter of the largest circle on the left-side diagram and the sum of four diameters (8 radii) of the smaller circles. This means that the radius of the largest circle is half of the length of one side of the square, or 2 diameters (4 radii) of the smaller circles. Letting "R" denote the radius of the largest circle, then R = 4r. The are of the largest circle is pi (R^2). substituting the "R" for 4r, the area of the largest circle is 16(pi (r^2)). This means that the are that the small circles occupy together and the large circle occupies are equal within two congruent squares, and the shaded area is thus the same area in both diagrams.

Considering the Radius of the First Circle as 4x Then the radius of Small Circle is x

then their area X number of circles

Pi(4x)^2 = 16x^2 Pi

Pi(x)^2 x 16 = 16 x^2 Pi

Theres no change so they are equal thus ,,

area of the same square minus

the same Area unshaded = The Same !

Rishabh Jain
Dec 23, 2015

Let the side of the square be equal to x

The radius of the big circle comes to be x/2 and hence the white area comes to be.
π(x/2)² = π[x²/4]

In the second square the diameter of each circle would be x/4 and hence the radius would be x/8 . Thus the area of the smaller circles would be π(x/ 8)² = π(x/64)

. Since there are 16 circles, the total white area would be

π(x²/64)16 = πx²/4

Since the white area in both the squares is the same, the shaded area wold also be same (obviously :P). Thanks

Jaype Sanchez
Sep 28, 2015

Depends on: 1) the area of the square, 2) the size of the circle, 3) number of circles inside the square, 4) area of the circles and 5) the area the circles occupy. Since the first square has bigger circle than the other square, it doesn't mean that the area of the shaded region on the first square is bigger, so the answer is e q u a l equal .

This question correct answer is equal area

By observing the two pictures clearly we can say that the region of red is equal to the region of blue region, but may of us confuse and answered blue:)

Krishna Garg
May 26, 2015

We calculate areas of circles with in given squares first,witch is are equal in both cases.Therefore,area left out side with in both squares will be same.Ans.

Shehan Melanka
May 24, 2015

It is very weird that we all think alike and most of us got the right answer its at times like this I understand how similar we all are

Indeed we are!

Chung Kevin - 6 years ago
Bhanu Sarva
May 20, 2015

Assuming 'a' is the side of the square, Blue shaded area (aXa)- ( PiXaXa/4), and so is the red shaded area! That is simplify
(aXa) - 16(PiX(a/8)X(a/8) to get the same thing!

well I can't insert the symbol for 'Pi' :)

Henrique Vendrami
May 19, 2015

Both areas have the same size, cause the little circles anda the bigger circle used to ocup the same area in the square

How can we tell that?

Chung Kevin - 6 years ago

consider the first firgure calc the relative area of the circle to that of the square which results to 78.5% and do the same for next figure, will get same results.

Sav Summer
Apr 18, 2015

If you only look at one circle in the figure of many circles and imagine a square described of one circle, the ratio of only square area to the circle will not change. And since the two figures have the same total are the ratio will be the same.

That's a simple way of seeing that the ratio of areas is the same!

Chung Kevin - 6 years ago
Sheraz Ahmad
Mar 15, 2015

so simple. as we can see that the small circles are in large numbers and they have cover more area than the others. but confusion is there that we think they are not shaded but actually they are shaded with white color.

No, it has nothing to do with "shaded in white color". The amount of blue shading is equal to the amount of red shading. After all, the answers are "blue region", "red region", and have nothing to do with "white region".

Chung Kevin - 6 years, 2 months ago

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