Fibonacci Reverse Calculation?

Hi All!

I was reading Dr Knotts entries about Binets formula on his fibonacci page
http://www.maths.surrey.ac.uk/hosted-sites/R.Knott/Fibonacci/fibFormula.html

I entered the formula into an Excel spreadsheet to see how this would work ;

Binet Formula
Fib(n) =  (1.6180339^n – (–0.6180339..)^n) / 2.236067977..

Its fascinating to see how I can enter any number for (n) and see the
correct fibonacci number returned!

I have a question though, I am a newbie computer programmer using
VB and Python and my math skills are average at best but I am very
interest and curious about math and especially fibonacci numbers!

Say I wanted to reverse and change the process.

I have a very large fibonacci number and I want to know what the 
fibonacci number is at a specified position BEFORE it, how would
I compute this, what would the formula be?

So for example I have the 301 st fibo number of ;

359579325206583560961765665172189099052367214309267232255589801

And I want to be able to find what the fibonacci value would be (n) numbers
BEFORE this... so I might want to know what the fibo value would be 50
numbers before or the 251st fibo number.  How would I do that?


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