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

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Re: The formula for the surface of a sphere is S=4πr^2. Two lead balls [#permalink]
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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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]
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 [#permalink]
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First we have to determine the sum of the surface areas of the two smaller balls.
1. 4∏3^2 = 4*∏*9 = 36∏
2. 4∏6^2 = 4*∏*36=144∏
144∏+36∏= 180∏
Now we can determine the radius
180∏/4= 45∏

√45 = 5*3*3 = 3√5
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Re: The formula for the surface of a sphere is S=4r^2. Two lead balls [#permalink]
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Re: The formula for the surface of a sphere is S=4r^2. Two lead balls [#permalink]
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