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


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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