Equation for $y'$ from $\frac{y'}{ [1+(y')^2]^{1/2}} = c$  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. 

Square both sides, solve for ${y^{\prime}}^2$ and then take the square root.
