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Stargazing Problem about calculating focal length of a telescope

  1. Jul 21, 2012 #1
    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?

    Attached Files:

  2. jcsd
  3. Jul 22, 2012 #2


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    Staff Emeritus
    Science Advisor

    Are you asking how they mathematically got from the previous equation to the last one?
  4. Jul 23, 2012 #3
    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...
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