# Finding the outer diameter of an annulus

#### linapril

##### New member
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 cm2?

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

#### Reckoner

##### Member
Re: Help with geometry question needed

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 cm2?

I have been trying to solve this for ages, but failed so far... Can someone help?
Let's first make sure that the units are consistent. $28\text{ mm} = 2.8\text{ cm}.$

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 $r.$

#### linapril

##### New member
Re: Help with geometry question needed

Thank you!