In a book there is a derivation for y' that comes from
\frac{y'}{[1+(y')^2]^{1/2}} = c,
where $c$ is a constant. The result they had was
y' = \sqrt{\frac{c^2}{1-c^2}}.
How did they get this? I tried expanding the square, and other tricks, I cant seem to get their result.