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Which of the following describe this formula correctly: $S = 2\pi rh$

1.S is jointly proportional to radius and height

2.S is directly proportional to radius and height or

3.S varies directly with radius and height.

I am leaning towards choice 3 but I am not 100% positive.

This is stuff on variation that might help. k is a constant

"S varies as x" means S = kx  "S varies jointly as x and y" means S = kxy  "S varies as x + y" means S = k(x + y)  "S varies inversely as x" means S = k/x 
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    For what it's worth, I would use all three of those sentences to describe the relationship in question. I don't really see why your book is teaching you this.2011-05-09

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The verb "describe" in the question is in the plural. That would seem to imply that any number of answers can be selected. As Chris noted in the comments, all three sentences are OK (assuming an appropriate definition of the terms used).