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The figure represents five concentric quarter-circles. The length of  [#permalink]

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Difficulty:   35% (medium)

Question Stats: 44% (02:10) correct 56% (02:22) wrong based on 71 sessions

### HideShow timer Statistics The figure represents five concentric quarter-circles. The length of the radius of the largest quarter-circle is x. The length of the radius of each successively smaller quarter-circle is one less than that of the next larger quarter-circle. What is the combined area of the shaded regions (black), in terms of x?

A. $$\pi(x^2-4x+10)$$

B. $$\frac{\pi}{2}(x^2-4x+10)$$

C. $$\frac{\pi}{4}(x^2-4x+10)$$

D. $$\frac{\pi}{8}(x^2-4x+10)$$

E. $$\frac{\pi}{16}(x^2-4x+10)$$

Attachment: Circles+.PNG [ 2.4 KiB | Viewed 12276 times ]

Guys - I neither have answer choices nor the OA. Can someone please help? This is how I am trying to solve it.
Area of the larger circle = pi $$x^2$$

Area of middle circle = pi (x-1)$$^2$$

Area of outer circle = pi (x-2)$$^2$$

Combined area should be the sum of the above 3. So when I do the maths I get combined area as

pi (3$$x^2$$ - 4x +5). But I am stuck after this.

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Re: The figure represents five concentric quarter-circles. The length of  [#permalink]

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enigma123 wrote:
The figure represents five concentric quarter-circles. The length of the radius of the largest quarter-circle is x. The length of the radius of each successively smaller quarter-circle is one less than that of the next larger quarter-circle. What is the combined area of the shaded regions (black), in terms of x?
Attachment:
Circles+.PNG

Guys - I neither have answer choices nor the OA. Can someone please help? This is how I am trying to solve it.
Area of the larger circle = pi $$x^2$$

Area of middle circle = pi (x-1)$$^2$$

Area of outer circle = pi (x-2)$$^2$$

Combined area should be the sum of the above 3. So when I do the maths I get combined area as

pi (3$$x^2$$ - 4x +5). But I am stuck after this.

Mind you, if you do have the options (as you will in GMAT), just plug in a value for x and solve. Working with numbers is much easier.
Radius of smallest circle is 1 and of largest is 5.

Required Area =$$\frac{1}{4}(\pi*25 - \pi*16 + \pi*9 - \pi*4 +\pi)$$
$$= 15\pi/4$$

In the options, just plug x = 5 and get your answer.
In the answer that Bunuel got above, if we plug x = 5, we get $$\frac{\pi(5^2 - 4*5 + 10)}{4} = 15\pi/4$$
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##### General Discussion
Math Expert V
Joined: 02 Sep 2009
Posts: 59587
Re: The figure represents five concentric quarter-circles. The length of  [#permalink]

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enigma123 wrote:
The figure represents five concentric quarter-circles. The length of the radius of the largest quarter-circle is x. The length of the radius of each successively smaller quarter-circle is one less than that of the next larger quarter-circle. What is the combined area of the shaded regions (black), in terms of x?
Attachment: circles.PNG [ 25.38 KiB | Viewed 12174 times ]

The radii of the 5 quarter-circles are: x (the largest red), x-1 (the largest white), x-2 (next red), x-3 (next white), and x-4 (the inner red), so 5 radii for 5 quarter-circles.

We want the sum of 3 red (black) regions (so without two white regions in the middle of them).

The area of the largest red region is 1/4 of the are of the largest circle minus 1/4 of the area of the circle with radius of x-1 (the largest white circle) --> $$\frac{\pi{x^2}}{4}-\frac{\pi{(x-1)^2}}{4}=\frac{\pi}{4}(x^2-(x-1)^2)=\frac{\pi}{4}(2x-1)$$;

The area of the next red region is 1/4 of the are of the circle with radius of x-2 (the next red circle) minus 1/4 of the area of the circle with radius of x-3 (the next white circle) --> $$\frac{\pi{(x-2)^2}}{4}-\frac{\pi{(x-3)^2}}{4}=\frac{\pi}{4}(2x-5)$$;

Finally, the are of the red region in the center with the radius of (x-4) is $$\frac{\pi{(x-4)^2}}{4}=\frac{\pi}{4}(x^2-8x+16)$$;

Sum: $$\frac{\pi}{4}(2x-1+2x-5+x^2-8x+16)=\frac{\pi}{4}(x^2-4x+10)$$.
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Re: The figure represents five concentric quarter-circles. The length of  [#permalink]

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The radius of the largest circle is x, so the radius of next is x-1 and so on so the area is
pi[(x^2-{x-1}^2)+({x-2}^2-{x-3}^2)+(x-4)^2]
on solving we get pi(x^2-4x+10)
since this is a quarter of a circle so the area is
1/4*pi(x^2-4x+10)
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Re: The figure represents five concentric quarter-circles. The length of  [#permalink]

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Using x=5, Area of shaded region

= $$\frac{1}{4} [π+(9π-4π)+(25π-16π)]$$
= $$\frac{15}{4}π$$
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Re: The figure represents five concentric quarter-circles. The length of  [#permalink]

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Hi Bunuel,

Would be great if you can add answer choices and OA to this question. You may add choices of your wish for the 4 incorrect choices in case the official ones are not available.

Warm Regards,
Pritishd
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Re: The figure represents five concentric quarter-circles. The length of  [#permalink]

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Pritishd wrote:
Hi Bunuel,

Would be great if you can add answer choices and OA to this question. You may add choices of your wish for the 4 incorrect choices in case the official ones are not available.

Warm Regards,
Pritishd

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_________________ Re: The figure represents five concentric quarter-circles. The length of   [#permalink] 20 Oct 2019, 20:50
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