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Manager  Joined: 06 Feb 2010
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In the figure shown, if the area of the shaded region is 3 t  [#permalink]

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20 00:00

Difficulty:   45% (medium)

Question Stats: 62% (01:46) correct 38% (02:05) wrong based on 514 sessions

### HideShow timer Statistics  In the figure shown, if the area of the shaded region is 3 times the area of the smaller circular region, then the circumference of the larger circle is how many times the circumference of the smaller circle?

A. $$4$$
B. $$3$$
C. $$2$$
D. $$\sqrt{3}$$
E. $$\sqrt{2}$$

Attachment: 1111220830.jpg [ 344.39 KiB | Viewed 28430 times ]
Attachment: Untitled2.png [ 13.24 KiB | Viewed 39605 times ]

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Originally posted by monirjewel on 11 Nov 2010, 09:09.
Last edited by Bunuel on 06 Dec 2017, 12:24, edited 2 times in total.
Renamed the topic, edited the question and added the OA.
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Re: In the figure shown, if the area of the shaded region is 3 t  [#permalink]

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3
10 In the figure shown, if the area of the shaded region is 3 times the area of the smaller circular region, then the circumference of the larger circle is how many times the circumference of the smaller circle?

A. $$4$$
B. $$3$$
C. $$2$$
D. $$\sqrt{3}$$
E. $$\sqrt{2}$$

The area of the shaded region is $$area_{shaded}=\pi{R^2}-\pi{r^2}$$ and the area of the smaller circle is $$area_{small}=\pi{r^2}$$. Given: $$\pi{R^2}-\pi{r^2}=3\pi{r^2}$$ --> $$R^2=4r^2$$ --> $$R=2r$$;

Now, the ratio of the circumference of the larger circle to the that of the smaller circle is $$\frac{C}{c}=\frac{2\pi{R}}{2\pi{r}}=\frac{{2r}}{{r}}=2$$.

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Re: In the figure shown, if the area of the shaded region is 3 t  [#permalink]

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1
circular shaded region = area of large circle - area of small circle = 3 * area of small circle
so area of large circle = 4 * area of small circle

that means radius oflarge circle = 2 * radius of small circle
so the cicumference of large circle is 2 times that of the small circle ---Option C
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Re: In the figure shown, if the area of the shaded region is 3 t  [#permalink]

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Bumping for review and further discussion.

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Re: In the figure shown, if the area of the shaded region is 3 t  [#permalink]

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3
Let the area of small circle = x

Area of shaded region = 3x

Total Area = x+3x = 4x

This means that Area of larger circle is 4 times Area of Small Circle;

which also means radius of larger circle is 2 times radius of small circle & so the circumference

So Answer = C = 2
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Re: In the figure shown, if the area of the shaded region is 3 t  [#permalink]

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Bunuel wrote: In the figure shown, if the area of the shaded region is 3 times the area of the smaller circular region, then the circumference of the larger circle is how many times the circumference of the smaller circle?

A. $$4$$
B. $$3$$
C. $$2$$
D. $$\sqrt{3}$$
E. $$\sqrt{2}$$

The area of the shaded region is $$area_{shaded}=\pi{R^2}-\pi{r^2}$$ and the area of the smaller circle is $$area_{small}=\pi{r^2}$$. Given: $$\pi{R^2}-\pi{r^2}=3\pi{r^2}$$ --> $$R^2=4r^2$$ --> $$R=2r$$;

Now, the ratio of the circumference of the larger circle to the that of the smaller circle is $$\frac{C}{c}=\frac{2\pi{R}}{2\pi{r}}=\frac{{2r}}{{r}}=2$$.

HI Bunnel,

Have one doubt on this as you have mentioned here R = 2r. Now we put this in the pi*rsquare then for both circles the are we got is

4pir square = pi r square.

Now area is four times instead of 3 times. So please clarify this
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Re: In the figure shown, if the area of the shaded region is 3 t  [#permalink]

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Bunuel wrote: In the figure shown, if the area of the shaded region is 3 times the area of the smaller circular region, then the circumference of the larger circle is how many times the circumference of the smaller circle?

A. $$4$$
B. $$3$$
C. $$2$$
D. $$\sqrt{3}$$
E. $$\sqrt{2}$$

The area of the shaded region is $$area_{shaded}=\pi{R^2}-\pi{r^2}$$ and the area of the smaller circle is $$area_{small}=\pi{r^2}$$. Given: $$\pi{R^2}-\pi{r^2}=3\pi{r^2}$$ --> $$R^2=4r^2$$ --> $$R=2r$$;

Now, the ratio of the circumference of the larger circle to the that of the smaller circle is $$\frac{C}{c}=\frac{2\pi{R}}{2\pi{r}}=\frac{{2r}}{{r}}=2$$.

HI Bunnel,

Have one doubt on this as you have mentioned here R = 2r. Now we put this in the pi*rsquare then for both circles the are we got is

4pir square = pi r square.

Now area is four times instead of 3 times. So please clarify this

The stem says that the area of the shaded region is 3 times the area of the smaller circular region, not that the area of the larger circular region is 3 times the area of the smaller circular region.

Does this make sense?
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Re: In the figure shown, if the area of the shaded region is 3 t  [#permalink]

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monirjewel wrote:
Attachment:
Untitled2.png
In the figure shown, if the area of the shaded region is 3 times the area of the smaller circular region, then the circumference of the larger circle is how many times the circumference of the smaller circle?

A. $$4$$
B. $$3$$
C. $$2$$
D. $$\sqrt{3}$$
E. $$\sqrt{2}$$
Attachment:
1111220830.jpg

Algebraically
Let a = Small circle's area
Let s = Shaded region's area
Let A = Large circle's area

$$s = 3a$$
$$A = s + a$$
$$A = 3a + a$$
$$A = 4a$$

$$a = \pi r^2$$
$$A = \pi R^2$$

From above:
$$A = 4a$$
$$\pi R^2 = 4\pi r^2$$
$$R^2 = 4r^2$$
$$\sqrt{R^2}=\sqrt{4r^2}$$
$$R = 2r$$
*

Circumference, Large and Small
C = $$2\pi R$$, and c = $$2\pi r$$
$$R = 2r$$, so
$$\frac{C}{c}=\frac{2\pi (2r)}{2\pi r}= \frac{2r}{r}=2$$

The large circle's circumference is two times the small circle's circumference.

Numbers and algebra

Large circle's area: Small circle's area?

Small circle's area = x
Large circle's area = 3x + x = 4x

Large circle circumference/ small circle's circumference?

Let Small circle's radius $$r = 1$$
Area of Small: $$\pi r^2 = \pi$$
Area of Large = (4x) = $$(4*\pi) = 4\pi$$

$$4\pi =\pi R^2$$
$$R = 2$$

Circumference, Small: $$2 \pi r = 2 \pi$$
Circumference, Large: $$2 \pi R = 4 \pi$$

$$\frac{Large}{Small}=\frac{4\pi}{2\pi} = 2$$

The large circle's circumference is two times the small circle's circumference.

**OR
$$A = 3a + a$$
$$A - 3a = a$$ AND $$A = \pi R^2$$

$$\pi R^2 - 3\pi r^2=\pi r^2$$
$$\pi (R^2 - 3r^2)=\pi r^2$$
$$R^2 - 3r^2 = r^2$$
$$R^2 = 4r^2$$ Take square roots:
$$R = 2r$$

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In the figure shown, if the area of the shaded region is 3 t  [#permalink]

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Bunuel,

The area of the shaded region is areashaded=πR2−πr2areashaded=πR2−πr2 and the area of the smaller circle is areasmall=πr2areasmall=πr2.

I write this like: πR^2=3*πr^2 (as is say 3 times)

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Re: In the figure shown, if the area of the shaded region is 3 t  [#permalink]

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amitpandey25 wrote:
Bunuel,

The area of the shaded region is areashaded=πR2−πr2areashaded=πR2−πr2 and the area of the smaller circle is areasmall=πr2areasmall=πr2.

I write this like: πR^2=3*πr^2 (as is say 3 times)

The stem says that the area of the shaded region is 3 times the area of the smaller circular region, not that the area of the larger circular region is 3 times the area of the smaller circular region.
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GMAT 1: 800 Q51 V49 GRE 1: Q170 V170 Re: In the figure shown, if the area of the shaded region is 3 t  [#permalink]

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Hi All,

This question can be solved by TESTing VALUES.

We're told that the area of the shaded region is 3 TIMES the area of the central circle...

Area of center = 1
Area of shaded region = 3(1) = 3
Area of FULL CIRCLE = 1+3 = 4

With those values....
Radius of FULL CIRCLE = 2

The question asks how many times the circumference of the full circle is to the smaller circle...

Circumference of small circle = 2pi
Circumference of full circle = 4pi

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Re: In the figure shown, if the area of the shaded region is 3 t  [#permalink]

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Bunuel wrote: In the figure shown, if the area of the shaded region is 3 times the area of the smaller circular region, then the circumference of the larger circle is how many times the circumference of the smaller circle?

A. $$4$$
B. $$3$$
C. $$2$$
D. $$\sqrt{3}$$
E. $$\sqrt{2}$$

The area of the shaded region is $$area_{shaded}=\pi{R^2}-\pi{r^2}$$ and the area of the smaller circle is $$area_{small}=\pi{r^2}$$. Given: $$\pi{R^2}-\pi{r^2}=3\pi{r^2}$$ --> $$R^2=4r^2$$ --> $$R=2r$$;

Now, the ratio of the circumference of the larger circle to the that of the smaller circle is $$\frac{C}{c}=\frac{2\pi{R}}{2\pi{r}}=\frac{{2r}}{{r}}=2$$.

Hi Bunuel,

Can you share problems that are similar to this?

Thanks.
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Re: In the figure shown, if the area of the shaded region is 3 t  [#permalink]

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1
shivamtibrewala wrote:
Bunuel wrote: In the figure shown, if the area of the shaded region is 3 times the area of the smaller circular region, then the circumference of the larger circle is how many times the circumference of the smaller circle?

A. $$4$$
B. $$3$$
C. $$2$$
D. $$\sqrt{3}$$
E. $$\sqrt{2}$$

The area of the shaded region is $$area_{shaded}=\pi{R^2}-\pi{r^2}$$ and the area of the smaller circle is $$area_{small}=\pi{r^2}$$. Given: $$\pi{R^2}-\pi{r^2}=3\pi{r^2}$$ --> $$R^2=4r^2$$ --> $$R=2r$$;

Now, the ratio of the circumference of the larger circle to the that of the smaller circle is $$\frac{C}{c}=\frac{2\pi{R}}{2\pi{r}}=\frac{{2r}}{{r}}=2$$.

Hi Bunuel,

Can you share problems that are similar to this?

Thanks.

Shaded Region Problems from our Special Questions Directory.
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Re: In the figure shown, if the area of the shaded region is 3 t  [#permalink]

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monirjewel wrote: In the figure shown, if the area of the shaded region is 3 times the area of the smaller circular region, then the circumference of the larger circle is how many times the circumference of the smaller circle?

A. $$4$$
B. $$3$$
C. $$2$$
D. $$\sqrt{3}$$
E. $$\sqrt{2}$$

Attachment:
1111220830.jpg
Attachment:
Untitled2.png

If we let A = the radius of the larger circle and B = the radius of the smaller circle, then we can create the equation:

(A^2 - B^2)π = area of shaded region

Area of the smaller circle = πB^2; thus:

(A^2 - B^2)π = 3πB^2

A^2 - B^2 = 3B^2

A^2 = 4B^2

A = 2B

Since the radius of the larger circle can be expressed as 2B, the circumference of the larger circle is 4Bπ, and the circumference of the smaller circle is 2Bπ, so the circumference of the larger circle is twice that of the smaller circle.

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Re: In the figure shown, if the area of the shaded region is 3 t  [#permalink]

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