In this derivation, how is it possible that we get a constant of integration "A" as shown in the image, included in the final derivation? I have included what I understand from the derivation process but I don't see how A fits in the final answer. Could it be related to the fact that after all $K=\dfrac{Kf}{Kb}$ and when treating $Kf$ only $Kb$ is kept constant?
I think I figured it out. I if we take into account the upper and lower limits of integration we end up with a -log(kf1) which then is taken to be the constant of integration A. Thanks!