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Circles A, B and C are externally tangent to each other, and internall

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Circles A, B and C are externally tangent to each other, and internall  [#permalink]

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New post 21 Mar 2019, 05:05
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A
B
C
D
E

Difficulty:

  85% (hard)

Question Stats:

11% (03:50) correct 89% (02:29) wrong based on 9 sessions

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Circles A, B and C are externally tangent to each other, and internally tangent to circle D. Circles B and C are congruent. Circle A has radius 1 and passes through the center of D. What is the radius of circle B?


(A) 2/3

(B) \(\frac{\sqrt{3}}{2}\)

(C) 7/8

(D) 8/9

(E) \(\frac{1 + \sqrt{3}}{3}\)


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Circles A, B and C are externally tangent to each other, and internall  [#permalink]

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New post 26 Mar 2019, 09:03
[quote="Bunuel"]Circles A, B and C are externally tangent to each other, and internally tangent to circle D. Circles B and C are congruent. Circle A has radius 1 and passes through the center of D. What is the radius of circle B?


(A) 2/3

(B) \(\frac{\sqrt{3}}{2}\)

(C) 7/8

(D) 8/9

(E) \(\frac{1 + \sqrt{3}}{3}\)

Join the lines as shown...
Now DE = 2, double the radius of circle A. So, BD = DE-BE=2-r
Take triangle BCD, \(CD=\sqrt{(2-r)^2-r^2}=\sqrt{4+r^2-4r-r^2}=2\sqrt{1-r}\)..
Now, take triangle ABC, \(AB^2=(BC)^2+(AD+DC)^2..........(1+r)^2=r^2+(1+2\sqrt{1-r})^2....1+2r+r^2=r^2+1+4(1-r)+4\sqrt{1-r}....9r^2=8r\). r cannot be 0, so 9r=8 or r=\(\frac{8}{9}\)


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Circles A, B and C are externally tangent to each other, and internall   [#permalink] 26 Mar 2019, 09:03
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Circles A, B and C are externally tangent to each other, and internall

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