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Limit of ratios of consecutive Fibonacci terms

Question

Let FnF_n denote the Fibonacci numbers. The limit of Fn+1/FnF_{n+1}/F_n as n→∞n\to\infty can be written as a+bc\dfrac{a+\sqrt b}{c} with a,b,ca,b,c pairwise coprime positive integers. What is abcabc?

Short answer

10. Divide the recurrence by the middle term: the ratio rn=Fn+1/Fnr_n=F_{n+1}/F_n obeys rn=1+1/rn−1r_n=1+1/r_{n-1}, so its limit LL solves L2=L+1L^2=L+1. The positive root is the golden ratio 1+52\tfrac{1+\sqrt5}{2}, so abc=1⋅5⋅2=10abc=1\cdot5\cdot2=10.

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