Which constant is it?

Algebra Level 3

So, suppose we have this great infinite fraction:

x = 1 + 1 1 + 1 1 + 1 1 + . . . x=1+\frac{1}{1+\frac{1}{1+\frac{1}{1+...}}}

Which mathematical constant is x x equal to?

e e ϕ \phi i i π \pi

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2 solutions

If we observe the problem closely,we can see that the whole question is embedded in itself.So we can write the problem as: x = 1 + 1 x x=1+\frac{1}{x} Rearranging this, we get: x = 1 + 1 x x 2 x 1 = 0 x=1+\frac{1}{x}\rightarrow x^2-x-1=0 Putting in the respective values in the quadratic formula,we get; ( 1 ) ± ( 1 ) 2 4 ( 1 ) ( 1 ) 2 ( 1 ) \frac{-(-1)\pm\sqrt{(-1)^2-4(1)(-1)}}{2(1)} 1 ± 1 + 4 2 \frac{1\pm\sqrt{1+4}}{2} 1 ± 5 2 \frac{1\pm\sqrt{5}}{2} Now as the infinite continued fraction is positive,we discard the negative root and the answer is 1 + 5 2 \frac{1+\sqrt{5}}{2} .Now this number is a very special number;it is... T h e G o l d e n R a t i o ! ! ! \LARGE{The\;Golden\;Ratio!!!} Which is denoted by ϕ \phi so answer is ϕ \boxed{\phi}

Robert Haywood
Nov 18, 2014

The fraction doesn't ever change, so that means we can say x = 1 + 1 x x=1+\frac{1}{x}

Multiplying by x x on both sides gives us x 2 = 1 + x x ^ 2=1+x .

Subtracting x x and then 1 1 from both sides will then give us the quadratic x 2 x 1 = 0 x ^ 2-x-1=0 .

From this, we know a = 1 a=1 , b = 1 b=-1 , and c = 1 c=-1 .

Plugging this into the quadratic formula for x becomes this: x = 1 ± 5 2 x=\frac{1\pm\sqrt{5}}{2} . If we choose to add 1, we get this:

x = 1 + 5 2 x=\frac{1+\sqrt{5}}{2}

Sound familiar? It should, because the above is also equal to phi, so ϕ \phi is the answer!

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