# Homework Help: Surface Area of Hollowed Hemisphere - Basic Geometry

1. Mar 22, 2012

### anniecvc

I need to find the total surface area of the hollowed out hemisphere (picture attached), with an inner diameter of 1.86 cm and what looks like an outer radius (also what I am assuming as height) of 1.25 cm.

Surface area of a sphere is 4∏r2, so half the sphere is 2∏r2.
Since the outer hemisphere has a radius of 1.25 cm, its SA is 2∏(1.25cm)2. The inner hemisphere has a diameter of 1.86cm, therefore a radius of .93 cm. Inner hemisphere then as a SA of 2∏(.93cm)2. Adding the two SA, I get:

2∏(1.252+.932) = apprx. 15.25 cm2.

However, the back of the book says the correct answer is apprx 17.44 cm2. So what am I doing wrong???

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2. Mar 22, 2012

### tiny-tim

hi anniecvc!

erm

what about the ring on the top?

3. Mar 22, 2012

### anniecvc

thank you tiny tim! i included it before it and the numbers didn't work out, but now it does, so i must've made a simple calculation error. thanks again.