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If the circle below has centre O and length of the arc RST is 18pi

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Intern
Joined: 29 Aug 2010
Posts: 15

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If the circle below has centre O and length of the arc RST is 18pi [#permalink]

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28 Oct 2017, 22:01
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If the circle below has centre O and length of the arc RST is 18 pi, what is the perimeter of the
region RSTU?

(A) 12 pi+ 18
(B) 12 pi+ 27
(C) 18 pi+ 27
(D) 18 pi+ 54
(E) 18 pi+ 108
[Reveal] Spoiler: OA

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Circle Centre.PNG [ 5.45 KiB | Viewed 365 times ]

Kudos [?]: 15 [0], given: 124

Math Expert
Joined: 02 Aug 2009
Posts: 5341

Kudos [?]: 6107 [0], given: 121

Re: If the circle below has centre O and length of the arc RST is 18pi [#permalink]

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28 Oct 2017, 22:24
shibbirahamed wrote:
If the circle below has centre O and length of the arc RST is 18 pi, what is the perimeter of the
region RSTU?

(A) 12 pi+ 18
(B) 12 pi+ 27
(C) 18 pi+ 27
(D) 18 pi+ 54
(E) 18 pi+ 108

hi

length of arc RST = $$\frac{60}{360} * perimeter= \frac{2*pi*r}{6}=18*pi......r=54..$$
triangle is EQUILATERAL triangle as two sides are radius so RT = radius = 54

perimeter = 18*pi +54

D
_________________

Absolute modulus :http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372
Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html

Kudos [?]: 6107 [0], given: 121

Intern
Joined: 29 Aug 2010
Posts: 15

Kudos [?]: 15 [0], given: 124

Re: If the circle below has centre O and length of the arc RST is 18pi [#permalink]

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28 Oct 2017, 22:40
Length of the arc RST (which subtends $$60^{\circ}$$ at the center) = 2$$\pi$$rX$$\frac{60}{360}$$, where r is the radius
of the circle. Thus, we have:
2$$\pi$$rX$$\frac{60}{360}$$ = 2$$\pi$$r
=> r = 54

In triangle TOR, we have: RO = TO = radius of the circle

Hence, we have:
$$\angle$$OTR = $$\angle$$ORT = $$\frac{180^{\circ}-60^{\circ}}{2}$$ = $$60^{\circ}$$

Thus, triangle TOR is equilateral.
Thus, we have: RT = TO = RO = radius of the circle = 54.
Thus, perimeter of the region RSTU = (18$$\pi$$+54).
The correct answer is option D.

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Intern
Joined: 24 Jun 2017
Posts: 6

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Re: If the circle below has centre O and length of the arc RST is 18pi [#permalink]

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04 Dec 2017, 03:16
shibbirahamed wrote:
Length of the arc RST (which subtends $$60^{\circ}$$ at the center) = 2$$\pi$$rX$$\frac{60}{360}$$, where r is the radius
of the circle. Thus, we have:
2$$\pi$$rX$$\frac{60}{360}$$ = 2$$\pi$$r
=> r = 54

In triangle TOR, we have: RO = TO = radius of the circle

Hence, we have:
$$\angle$$OTR = $$\angle$$ORT = $$\frac{180^{\circ}-60^{\circ}}{2}$$ = $$60^{\circ}$$

Thus, triangle TOR is equilateral.
Thus, we have: RT = TO = RO = radius of the circle = 54.
Thus, perimeter of the region RSTU = (18$$\pi$$+54).
The correct answer is option D.

how did u arrive that 2 angles would be equal??

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VP
Joined: 22 May 2016
Posts: 1117

Kudos [?]: 397 [0], given: 640

If the circle below has centre O and length of the arc RST is 18pi [#permalink]

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04 Dec 2017, 06:06
shibbirahamed wrote:
If the circle below has centre O and length of the arc RST is 18 pi, what is the perimeter of the
region RSTU?

(A) 12 pi+ 18
(B) 12 pi+ 27
(C) 18 pi+ 27
(D) 18 pi+ 54
(E) 18 pi+ 108

The sector is a fraction of the circle; the sector's central angle is a fraction of the circle's 360°.

Use that fraction, and the given arc length, to find circumference. Circumference yields radius, which we need for the sides of the triangle.

Sector as fraction of circle
$$\frac{Part}{Whole}=\frac{60}{360}=\frac{1}{6}$$

The sector is 1/6 of the circle. Its arc length is 1/6 of circumference.

Radius: Length of arc RST = 1/6 circumference
$$18π = 2πr * \frac{1}{6}$$
$$54πr = πr$$
, so $$r = 54$$

Triangle side lengths
Angles opposite equal sides are equal.
Sides OR and OT are radii: OR = OT
Triangle vertices at R and at T are angles opposite those equal sides.
So triangle vertex ∠R = vertex ∠T

One angle, at vertex O, is 60°
Let the other two equal angles = x
(2x + 60 = 180) --> (2x = 120)
x = 60
The triangle is equilateral (60°-60°-60°).
Two sides each = length of radius = 54
Third side, RT, must = 54

Perimeter of region RSTU
Arc RST + ∆ side RT
18π + 54

Kudos [?]: 397 [0], given: 640

If the circle below has centre O and length of the arc RST is 18pi   [#permalink] 04 Dec 2017, 06:06
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