I'm reading a physics book( "Einstein's gravity in a nutshell" Zee) and at page 126 the author says: (dr/dl)^2 + a^2 / r^2 = 1 gives r^2 = l^2 + a^2 where we absorbed an integration constant into l by setting l=0 when r=a. Can someone explain what's going on here? How do I have to "massage" the first equation to make an integration that gives the second equation? What does Zee mean with absorbing an integration constant? Thanks for any respons!