Question

First-order non-linear diff

Answer 1

\[A = y(x) \sqrt{1 + ({dy \over dx})^2 } \]
where A is a constant

Not really sure where to start...
http://www.wolframalpha.com/input/?i=+A%3Dysqrt%281%2B%28dy%2Fdx%29^2%29

Answer 2

@Zarkon @Chlorophyll

Answer 3

@satellite73