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Fibonacci - so many formulas, so few time ...
Given that its not a Mathcad questions and probably also can't be done/proven with Mathcad, I guess chances are rather low that anyone will present a formal proof here.
Maybe you find here
http://www.maths.surrey.ac.uk/hosted-sites/R.Knott/Fibonacci/fibFormulae.html
and/or in the reference section at the end something of interest.
Maybe you are especially interested at Vajda
https://www.amazon.com/exec/obidos/ASIN/0486462765/fibonacnumbersan
or Johnson
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