Shouldn't it be infinitely continued?

Algebra Level 3

x = 4 1 1 1 4 1 1 1 4 1 1 1 4 1 1 1 x x=4-\frac{1}{1-\dfrac{1}{4-\dfrac{1}{1-\dfrac{1}{4-\dfrac{1}{1-\dfrac{1}{4-\dfrac{1}{1-\dfrac{1}{x}}}}}}}} Solve for x x .


The answer is 2.

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2 solutions

Jaydee Lucero
Jul 2, 2017

Since x = 4 1 1 1 4 1 1 1 4 1 1 1 4 1 1 1 x x=4-\frac{1}{1-\dfrac{1}{4-\dfrac{1}{1-\dfrac{1}{4-\dfrac{1}{1-\dfrac{1}{4-\dfrac{1}{1-\dfrac{1}{x}}}}}}}} repeatedly substituting this to the x x in the continued fraction leads us to the infinite continued fraction x = 4 1 1 1 4 1 1 1 4 1 1 1 4 1 1 1 x=4-\frac{1}{1-\dfrac{1}{\color{#D61F06}{4-\dfrac{1}{1-\dfrac{1}{4-\dfrac{1}{1-\dfrac{1}{4-\dfrac{1}{1-\dfrac{1}{\cdots}}}}}}}}} By passing to the limit, we see that this equation is equivalent to x = 4 1 1 1 x x = 4-\frac{1}{1-\dfrac{1}{\color{#D61F06}{x}}} Solving this equation, we get x = 4 1 1 1 x = 4 1 x 1 x = 4 x x 1 x ( x 1 ) = x 2 x = 4 ( x 1 ) x = 3 x 4 x 2 4 x + 4 = ( x 2 ) 2 = 0 x = 2 \begin{aligned} x &= 4-\frac{1}{1-\dfrac{1}{x}} = 4 - \frac{1}{\dfrac{x-1}{x}} = 4 - \frac{x}{x-1} \\ x(x-1) = x^2 - x &= 4(x - 1) - x = 3x - 4 \\ x^2 - 4x + 4 = (x-2)^2 &= 0 \\ x &= \boxed{2} \end{aligned}

Marta Reece
Jul 2, 2017

I did it by brute force, starting from the bottom and evaluating the fraction. It took less than a page.

The verification that 2 is the answer can be done the same way, but is far easier.

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