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In the figure above, the vertices of rectangle ABCD lie on a circle wi

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In the figure above, the vertices of rectangle ABCD lie on a circle wi  [#permalink]

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New post 21 Nov 2017, 23:30
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In the figure above, the vertices of rectangle ABCD lie on a circle with center O. If BD = 8, CD = 4 and x = 120, what is the perimeter of the shaded region?

(A) 8π/3 + 2√3
(B) 8π/3 + 4√2
(C) 8π/3 + 4√3
(D) 8π + 2√3
(E) 8π + 4√3

Attachment:
2017-11-22_1022_003.png
2017-11-22_1022_003.png [ 8.37 KiB | Viewed 744 times ]

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Re: In the figure above, the vertices of rectangle ABCD lie on a circle wi  [#permalink]

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New post 22 Nov 2017, 00:32
Answer is C:

We can calculate the Length of arc from the relationship between the angle subtended at the centre and the net circumference of the circle.

If angle X. Then the perimeter of arc is (X/360) X 2*pi*r.

We know that O is the centre and BD is 8. CD is 4.
There are 2 ways to find out the length of BC. Using trigonometry in Triangle OBC or using pythagorean theorem in Triangle BCD.

Arc length + BC is the perimeter.
Which is 8*pi*1/3 + 4 sqrt3.

Hence C


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Re: In the figure above, the vertices of rectangle ABCD lie on a circle wi  [#permalink]

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New post 22 Nov 2017, 00:47
Perimeter of shaded region = length of arc BC + line BC
length of arc BC = 120/360 * 2*P*r=1/3*2*P*4=8/3*P
line BC = (8^2-4^2)^1/2=4*3^1/2
substituting:
Perimeter of shaded region = length of arc BC + line BC = 8/3*P + 4*3^1/2
Answer C
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In the figure above, the vertices of rectangle ABCD lie on a circle wi  [#permalink]

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New post 27 Nov 2017, 08:06
1
Bunuel wrote:
Image
In the figure above, the vertices of rectangle ABCD lie on a circle with center O. If BD = 8, CD = 4 and x = 120, what is the perimeter of the shaded region?

(A) 8π/3 + 2√3
(B) 8π/3 + 4√2
(C) 8π/3 + 4√3
(D) 8π + 2√3
(E) 8π + 4√3

Attachment:
2017-11-22_1022_003.png

Perimeter of shaded region =
(Sector arc length) + (rectangle length BC)

Sector Arc Length

\(\frac{SectorAngle}{360}=\frac{Part}{Whole}=\frac{ArcLength}{Circumference}\)

Sector angle = x = 120°
r = 1/2 diagonal = 8/2 = 4
Circumference = \(2\pi r = 8\pi\)

\(\frac{120}{360}=\frac{1}{3}=\frac{ArcLength}{8\pi}\)

Arc Length\(*(3) = (8\pi)\)
Arc Length = \(\frac{8\pi}{3}\)

Length BC of rectangle

Length of BC = x
\(BC^2 + x^2 = AC^2\)
\(16 + x^2 = 64\)
\(x^2 = 48\)
\(\sqrt{x^2} =\sqrt{16*3}\)
\(x = 4\sqrt{3}\)


Perimeter = \(\frac{8\pi}{3} + 4\sqrt{3}\)

Answer C
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Re: In the figure above, the vertices of rectangle ABCD lie on a circle wi  [#permalink]

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New post 27 Nov 2017, 12:37
Bunuel wrote:
Image
In the figure above, the vertices of rectangle ABCD lie on a circle with center O. If BD = 8, CD = 4 and x = 120, what is the perimeter of the shaded region?

(A) 8π/3 + 2√3
(B) 8π/3 + 4√2
(C) 8π/3 + 4√3
(D) 8π + 2√3
(E) 8π + 4√3

Attachment:
2017-11-22_1022_003.png


We see that the perimeter of the shaded region consists of side BC of the rectangle and arc BC of the circle. Let’s first determine the length of side BC of the rectangle.

Using the Pythagorean theorem, we have:

4^2 + b^2 = 8^2

16 + b^2 = 64

b^2 = 48

b = √48 = √16 x √3 = 4√3 = side BC

Since the diagonal BD = 8 = diameter of the circle, the circumference of the circle = 8π. Since arc BC corresponds to a central angle of 120 degrees, the length of arc BC = (120/360) x 8π = (1/3) x 8π = 8π/3.

Thus, the perimeter of the shaded region = 8π/3 + 4√3.

Answer: C
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Re: In the figure above, the vertices of rectangle ABCD lie on a circle wi  [#permalink]

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New post 27 Nov 2017, 21:25
genxer123 wrote:
Bunuel wrote:
Image
In the figure above, the vertices of rectangle ABCD lie on a circle with center O. If BD = 8, CD = 4 and x = 120, what is the perimeter of the shaded region?

(A) 8π/3 + 2√3
(B) 8π/3 + 4√2
(C) 8π/3 + 4√3
(D) 8π + 2√3
(E) 8π + 4√3

Attachment:
2017-11-22_1022_003.png

Perimeter of shaded region =
(Sector arc length) + (rectangle length BC)

Sector Arc Length



\(\frac{SectorAngle}{360}=\frac{Part}{Whole}=\frac{ArcLength}{Circumference}\)

Sector angle = x = 120°
r = 1/2 diagonal = 8/2 = 4
Circumference = \(2\pi r = 8\pi\)

\(\frac{120}{360}=\frac{1}{3}=\frac{ArcLength}{8\pi}\)

Arc Length\(*(3) = (8\pi)\)
Arc Length = \(\frac{8\pi}{3}\)

Length BC of rectangle

Length of BC = x
\(BC^2 + x^2 = AC^2\)
\(16 + x^2 = 64\)
\(x^2 = 48\)
\(\sqrt{x^2} =\sqrt{16*3}\)
\(x = 4\sqrt{3}\)


Perimeter = \(\frac{8\pi}{3} + 4\sqrt{3}\)

Answer C


I like the way you explain things stepwise and with proper formatting-sign of a good teacher, this makes students understand and learn rather than scratch their heads.
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Re: In the figure above, the vertices of rectangle ABCD lie on a circle wi &nbs [#permalink] 27 Nov 2017, 21:25
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