Problem about calculating focal length of a telescope

[Blacky&Imy]

Hello!
I am preparing an exam about optics and I can't understand a formula.my problem is at the end of the second page (the explanation is in Italian, but the formulas are easy to read for everybody).
I can't understand how in the last step

f=R/2 - (1+b) * r^2/4R etc they found -(1+b)

can somebody help me please?

 

[Drakkith]

Are you asking how they mathematically got from the previous equation to the last one?

 

[Blacky&Imy]

yes. I understand what is substituted in what, but I get a different sign in the second term, and I'd like to see all the passages...