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The formula for the surface of a sphere is S=4πr^2. Two lead balls

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The formula for the surface of a sphere is S=4πr^2. Two lead balls  [#permalink]

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New post 24 Oct 2015, 11:52
2
2
00:00
A
B
C
D
E

Difficulty:

  5% (low)

Question Stats:

86% (01:08) correct 14% (01:15) wrong based on 179 sessions

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The formula for the surface of a sphere is \(S=4πr^2\). Two lead balls have radii 3 inches and 6 inches, respectively. A third ball has surface area equal to the sum of the surface areas of the two smaller balls. What is the radius, in inches, of the third ball?

    (A) \(3\sqrt{10}\)
    (B) \(3\sqrt{5}\)
    (C) \(36π\)
    (D) \(45π\)
    (E) \(180π\)

It's not really a hard question, but I got it wrong because I didn't take enough time to read carefully the question. So i'll just leave it here for others to avoid my mistake.
Good luck ya'll

PS: Difficulty is probably under 600, but couldn't edit the tags

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Re: The formula for the surface of a sphere is S=4πr^2. Two lead balls  [#permalink]

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New post 24 Oct 2015, 12:28
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petocities
The correct answer is B.
The surface area of the bigger sphere is = 4π (3^2 +6^2) = 4πr^2
4π (9 + 36) = 4πr^2
45 = r^2
r = \sqrt{45}
= 3 \sqrt{5}
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Re: The formula for the surface of a sphere is S=4πr^2. Two lead balls  [#permalink]

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New post 25 Oct 2015, 09:45
erratic15 wrote:
petocities
The correct answer is B.
The surface area of the bigger sphere is = 4π (3^2 +6^2) = 4πr^2
4π (9 + 36) = 4πr^2
45 = r^2
r = \sqrt{45}
= 3 \sqrt{5}


Yeah, upon revising my calculations I got it right, but on my first time I stopped at 4π(9+36) = 180π. Since that's (E), I just selected it. That's the silly mistake that one has to avoid!
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Re: The formula for the surface of a sphere is S=4πr^2. Two lead balls  [#permalink]

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New post 27 Feb 2017, 10:51
Can please somebody explain how the translation from /sqrt(45) --> 3*/sqrt(5) works?
I got this one right but only because I canceled the 3 obvious wrong ones and then estimated what value r approximately has to be
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Re: The formula for the surface of a sphere is S=4πr^2. Two lead balls  [#permalink]

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New post 03 Mar 2017, 01:04
rasti12 wrote:
Can please somebody explain how the translation from /sqrt(45) --> 3*/sqrt(5) works?
I got this one right but only because I canceled the 3 obvious wrong ones and then estimated what value r approximately has to be


\(\sqrt{45}=\sqrt{9*5}=3\sqrt{5}\)

Hope it's clear.
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The formula for the surface of a sphere is S=4πr^2. Two lead balls  [#permalink]

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New post 07 Oct 2018, 07:18
Can someone please explain why the SA of the third lead ball is not 180π? I calculated the first SA as 36π and the second as 144π then added them together. What am I missing to get to 45π as the third ball's SA?
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The formula for the surface of a sphere is S=4πr^2. Two lead balls &nbs [#permalink] 07 Oct 2018, 07:18
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