Finding diameter of a sphere if all you know is surface area

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A test question we can't solve asks:

The surface area of a sphere is 86pi ft^2. What is the diameter of the sphere?

My daughter is solving by using the formula: SA= 4pir^2 to get the radius. She comes up with 4.64 and plugs that in to: pi x diameter = 2pi x radius. Her final answer is diameter = 9.28ft.

What is she doing wrong?[/quote][/code]
 
You're missing the pi.

2*r = 9.28 fr
2*pi*r = 29.134 ft

Don't forget the dessert!
 
Okay, thanks. The answer book says the correct answer is the square root of 86 ft. Are we still doing it right?
 
It seems a little confused. sqrt(86) ft is the Radius.
 
The surface area of a sphere is 86pi ft^2. What is the diameter of the sphere?


SA = 4pir^2
4pir^2 = 86pi
r = ((86pi)/(4pi))^(1/2) = 4.6368
d = 2r = 2*4.6368 = 9.274 ft, approx.
 
Great! Thanks for spelling it out. I really appreciate the help!
 
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