A B = 7 7 7 8 7 4 2 0 4 9 = 1 . 7 4 9
Which is bigger, A or B ?
Note : 7 7 7 8 7 4 2 0 4 9 = F 4 9 , the 49th Fibonacci number .
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Thanks! Don't you know, that is there a solution without using the golden ratio?
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Sorry, I don't know.
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.
I refused to use the note!
It's easy to show by hand that 1 7 4 > 1 0 5 . Equivalently, 1 7 4 8 1 7 4 9 > 1 7 4 8 9 × 1 7 4 9 9 × 1 7 4 9 9 × 1 7 4 9 B = ( 1 0 1 7 ) 4 9 B > > > > > > > 1 0 6 0 1 0 6 0 > 1 0 5 9 9 × 1 0 5 9 9 × 1 0 5 9 > 8 × 1 0 5 9 8 × 1 0 5 9 9 8 × 1 0 1 1 9 8 × 1 0 1 1 > A
Is it possible to prove with your solution, that f n < 1 , 7 n ?
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No, not possible. Since we want to prove for all (n), my solution is completely useless.
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Using computation by rounding , we have the n th Fibonacci number F n = ⌈ 5 φ n ⌋ , where φ = 2 1 + 5 is the golden ratio. Then A = F 4 9 = ⌈ 5 φ 4 9 ⌋ = ⌈ 5 ( 1 . 6 1 8 . . . ) 4 9 ⌋ < 1 . 7 4 9 = B , ⟹ B > A .