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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?

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# Stargazing Problem about calculating focal length of a telescope

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