Circs

Geometry Level 1

In the diagram, the yellow and red circles are identical and are both centered on the larger circle's diameter.

Which has a larger area?

Blue region Yellow circle They both have equal areas

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13 solutions

Aj Cham
Oct 30, 2017

The large circle is divided into four parts. The two small circles are half the radius of the large one, so have 1/4 the area each. The remaining 2 regions are identical, and together make up the other half of the total area, so must also be 1/4 each.

Moderator note:

When a length doubles between similar figures, the area will quadruple. To justify this, we can think of the similar figures as inscribed in squares; the proportion of the square "carved out" by the area will be the same in both cases.

In general, if a side length between similar figures is multiplied by S , S, then the area will be multiplied by S 2 . S^2.

So I know the numeracy behind this problem is easy and everything. But just for a sec imagine this: The red and the yellow circles are identical and are inscribed in the larger circle. What if I exchange the lower part of the yellow circle with the upper part of the red circle. Now, Ignore the lower half of the larger circle all together. If the blue area and the yellow are are really the same, why didn't the yellow area overlap the blue are entirely?

Akanksha Sharma - 2 months ago

Let y y be the area of yellow/red circle, b b be the area of the blue region and o o be the area of the orange region.

Let r r be the radius of the yellow/red circle, then the radius of the big circle is 2 r 2r .

So, we have

y = π r 2 y=\pi r^2

b = o = π ( 2 r ) 2 2 π r 2 2 = 4 π r 2 2 π r 2 2 = 2 π r 2 2 = π r 2 b=o=\dfrac{\pi (2r)^2-2\pi r^2}{2}=\dfrac{4\pi r^2-2\pi r^2}{2}=\dfrac{2\pi r^2}{2}=\pi r^2

\color{#D61F06} \large{\therefore} The four areas are equal. \boxed{\color{plum}\text{The four areas are equal.}}

To do this problem, I read the text. It said that the 2 circles have an equal size. This means that they probably have the same area too. I think to solve this problem you just need a good understanding of areas and logical reasoning works best. I would rate this problem a 1 because the text pretty much gave me the answer. I felt like it was too good to be true, SO EASY!!

Lucia Tiberio - 3 years, 7 months ago

πHe did the equation wrong at the beginning of the first equation instead of π(2r)^2 he should have put 2(π(2r)^2) because there are two separate regions that aren't the circles.

Ismael Soumahoro - 3 years, 7 months ago

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The equation is correct. To find the blue area, he is calculating the area of the larger circle, which has a radius which is double one of the small circles, hence (2r). Then he removes the area of the 2 small circles (2πr^2) and finally divides that remaining area in 2 parts because one is the blue area and the other one is the orange one.

Javier Morones - 3 years, 7 months ago
Just Someone
Nov 1, 2017

I don't get all of you guys, nor this question. What's with the complicated thinking process? The question says that they're identical, so what's there to calculate?

True. It says they’re identical, and it is placed in the center. Those two are your keywords :) meaning they are both equal.

Aya Mori - 3 years, 7 months ago

Huh! It says the yellow and red circles are identical.. Then it asks if the circle is the same area as the blue area.... Are you colour blind?

Kev Callan - 3 years, 7 months ago

I came to the answer by realizing that the radius of the big circle is the same as the radius of each smaller circle added together. Also, given the fact that both smaller circles are equal to each other and proportional to the large circle.

I have no training in math/geometry beyond 8th grade, so this purely logic and very basic geometry.

The complicated equations are over my head, but I am sure they are useful in demonstrating solutions for similar problems that are less straightforward.

I am guessing that they would prove that the same logic does not hold true if the inner circles are of unequal sizes. Again, it is only logic that brings me to that assumption, and I sanity check my assumptions by bringing questions to extremes: as one inner circle gets larger and the other gets smaller, the larger one approaches the maximum area of the outer circle, regardless of the fact that both radii added together would still equal the radius of the outer circle. So it was that sanity check that let me know that the inner circles being equal was relevant to determining the answer.

Amber Baker - 3 years, 7 months ago

You need to re-read the question and the answer choices... although the question is worded poorly.

Edit - ah, I see your point. The red and yellow circles are identical. Which has the larger area? Without adding any additional information the pronoun "which" has to refer back to the two previously mentioned circles (ie, red and yellow).

Chris Cheek - 3 years, 7 months ago
Maneck Daroga
Oct 21, 2017

Let radius of small circle be r So radius of big circle is 2r Area of yellow circle is (pi)r^2 and area of both smaller circles is 2 (pi)r^2 Now Area of big circle is 4 (pi)r^2 Hence area of blue and orange region together is also 2(pi)r^2 and as they are similar each blue / orange region area is (pi)r^2 So blue. yellow, red, orange all have same area

Let r = 1 and this collapses to Area of small circle = 1, and area of large circle = 4. Problem solved!

Ken Wilshire - 3 years, 7 months ago
Hunter Edwards
Oct 31, 2017

The are formula for a circle is π r 2 πr^2 . The definition of the radius (r) is d 2 \frac{d}{2} , with d d being the diameter. Therefore, if the two circles have an equal diameter, they must also have an equal area, as the diameter/radius of a circle is what defines the area.

Sumukh Bansal
Oct 30, 2017

You can solve it by looking at the options:-

It can't be blue as blue will be equal to orange.

It can't be yellow as yellow will be equal to red.

So answer is option 3 which is all are equal.

What do you mean?

Sumukh Bansal - 3 years, 7 months ago

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Yours solving strategy is false as all the areas are equal. If you dont believe check the other solutions as well.

Naren Bhandari - 3 years, 7 months ago

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My answer is option 3 all areas are same ,you can check it.

Sumukh Bansal - 3 years, 7 months ago

Love the deduction

Michelle Gapiza - 3 years, 7 months ago

That false all the area are equal.

Naren Bhandari - 3 years, 7 months ago

The diameter of one of the smaller circles is the radius of the big one, which can be applied horizontally and also vertically wich means that the area of the big circle is sum of 4 smaller circles. Or: 1small circle is 1/4 of the big one.

Davide Lombardi
Nov 4, 2017

The two smaller circles have a radius half than radius of big circle.Then the areas of small circles are 1/4 of the large one. Considering - Area(big circle)=1 - Area(red circle) + area(yellow circle) =1/2 then area(blue) = 1/4 = area(green region) also area(red circle)= area(yellow circle)=1/4

Ben Smith
Nov 3, 2017

Pi rsqu - 2(pi (r/2)squ)=pi rsqu-2pi(dsqu/4) =pi rsqu-2pirsqu/4=(4pi*rsqu)/4-2pirsqu/4 =2pirsqu/4 so the sum of the area of the small circles equals the remainder of the large cicle. Did u get that?

Moheb Eskaross
Nov 1, 2017

since the yellow (Y) section = red (R) section and they both add up to the diameter of the total, so area of of Yellow is (pi*(1/2r)^2) = pi * 1/4 of r^2 so area of Y is a 1/4 of the whole circle (yellow + blue + red + orange). Now we can see that the blue (B) section is equal to half the area of the whole circle - the half the area of Y and R (which is equal) so it can be written as: AreaB = 1/2(AreaWholeCircle) - AreaY. so so it is half - quartre of the whole circle = 1/4. so it is equal to Y

Siva Skandha
Oct 31, 2017

Both the circles (inside )are same in area and they are on diameter of outer circle. So the amount of loss of area in both regions is same...(it will have different areas when both inner circles are not in same line )

Jonathan Spirit
Oct 31, 2017

Let d be the diameter of the large circle. The area of a circle, expressed in terms of diameter, is A c i r c l e = π ( d 2 ) 2 = π 4 d 2 A_{circle} = \pi(\frac{d}{2})^2 = \frac{\pi}{4}d^2 . The area of a smaller circle with diameter d 2 \frac{d}{2} is A y = π 4 ( d 2 ) 2 = π 16 d 2 A_y = \frac{\pi}{4}(\frac{d}{2})^2 = \frac{\pi}{16}d^2 . The area of both circles is twice the area of one of them is A r + y = π 8 d 2 A_{r+y} = \frac{\pi}{8}d^2 . The rest of the circle's area is the area of the circle minus the area of the two circles is A b + o = π 4 d 2 π 8 d 2 = π 8 d 2 A_{b+o} = \frac{\pi}{4}d^2 - \frac{\pi}{8}d^2 = \frac{\pi}{8}d^2 . Regions b and o are identical, so we can divide that area by two to get A b = π 16 d 2 = A y A_b = \frac{\pi}{16}d^2 = A_y . The blue and yellow regions have the same area.

J Chapman
Oct 30, 2017

It states the two circles are identical. So obviously they have the same area. This is like the old question "which is heavier a pound of lead or a pound of feathers?"

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