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## Main Question or Discussion Point

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!

(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!