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- Thread starter linapril
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- Jun 16, 2012

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Let's first make sure that the units are consistent. $28\text{ mm} = 2.8\text{ cm}.$A pipe with a circular cross-section area has the inner diameter of 28 mm. How big should the pipe's outer diameter be, if the cross section area of the pipe's wall is 4.5 cm^{2}?

I have been trying to solve this for ages, but failed so far... Can someone help?

Suppose the outer radius is $r.$ The area of a cross section will be the area of the outer circle minus the area of the inner circle. So we have

\[4.5 = \pi r^2 - \pi(1.4)^2.\]

Solve for $r.$

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