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A = 7778742049 B = 1. 7 49 \begin{aligned} A & =7778742049 \\ \ \\ B & = 1.7^{49} \end{aligned}

Which is bigger, A A or B B ?

Note : 7778742049 = F 49 7778742049=F_{49} , the 49th Fibonacci number .

B > A B>A A > B A>B They are equal

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2 solutions

Chew-Seong Cheong
Jul 14, 2017

Using computation by rounding , we have the n n th Fibonacci number F n = φ n 5 F_n = \left \lceil \dfrac {\varphi^n}{\sqrt 5} \right \rfloor , where φ = 1 + 5 2 \varphi = \dfrac {1+\sqrt 5}2 is the golden ratio. Then A = F 49 = φ 49 5 = ( 1.618... ) 49 5 < 1. 7 49 = B A = F_{49} = \left \lceil \dfrac {\varphi^{49}}{\sqrt 5} \right \rfloor = \left \lceil \dfrac {(1.618...)^{49}}{\sqrt 5} \right \rfloor < 1.7^{49} = B , B > A \implies \boxed{B > A} .

Thanks! Don't you know, that is there a solution without using the golden ratio?

Áron Bán-Szabó - 3 years, 11 months ago

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Sorry, I don't know.

Chew-Seong Cheong - 3 years, 11 months ago

You can derive the nth Fibonacci number to be the Binet's formula by setting up the recurrence relation A(n) = A(n-1) + A(n-2). (or just apply induction ). You don't even have to know what Golden ratio is.

Pi Han Goh - 3 years, 11 months ago
Pi Han Goh
Jul 15, 2017

I refused to use the note!


It's easy to show by hand that 1 7 4 > 1 0 5 17^4 > 10^{5} . Equivalently, 1 7 48 > 1 0 60 1 7 49 > 1 7 48 > 1 0 60 > 1 0 59 9 × 1 7 49 > 9 × 1 0 59 9 × 1 7 49 > 9 × 1 0 59 > 8 × 1 0 59 9 × 1 7 49 > 8 × 1 0 59 B = ( 17 10 ) 49 > 8 9 × 1 0 11 B > 8 9 × 1 0 11 > A \begin{aligned} 17^{48} &>& 10^{60} \\ 17^{49} > 17^{48} &>& 10^{60} > 10^{59} \\ 9\times 17^{49} &>& 9\times 10^{59} \\ 9\times 17^{49} &>& 9\times 10^{59} > 8\times 10^{59} \\ 9\times 17^{49} &>& 8\times 10^{59} \\ B=\left( \frac{17}{10}\right)^{49} &>& \frac89 \times 10^{11} \\ B &>& \frac89 \times 10^{11} > A \end{aligned}

Is it possible to prove with your solution, that f n < 1 , 7 n f_n<1,7^n ?

Áron Bán-Szabó - 3 years, 11 months ago

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No, not possible. Since we want to prove for all (n), my solution is completely useless.

Pi Han Goh - 3 years, 11 months ago

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