Nested Root Simple Property

Algebra Level 1

x x x x x x x = ? \sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\cdots}}}}}}}\ = \ ?

In other words, what happens if we keep doing this forever? (Note that x 0 x\geqslant {0} .)

x 2 x^{2} x 3 2 x^{\frac{3}{2}} x 3 x^{3} x x

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4 solutions

Liviu Florescu
Feb 4, 2016

for me, the expression is equivalent to x 1 2 × x 1 4 × x 1 8 × x 1 16 . . . . x^{\frac{1}{2}} \times x^{\frac{1}{4}} \times x^{\frac{1}{8}} \times x^{\frac{1}{16}} .... which is equivalent to x 1 2 + 1 4 + 1 8 + 1 16 . . . x^{\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+\frac{1}{16}...} and we know 1 2 + 1 4 + 1 8 + 1 16 . . . \frac{1}{2}+\frac{1}{4}+\frac{1}{8}+\frac{1}{16}... converges to 1 1 , therefore the main expresion coverges to x 1 x^{1} which is x x

Nice job @Liviu Florescu . That's the same general train of thought I had when making the problem. :)

Drex Beckman - 5 years, 4 months ago

y = x x x x x x x x x x x . . . \color{#D61F06}{y}=\sqrt{\color{#3D99F6}{x}\color{#D61F06}{\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x...}}}}}}}}}}}}

y = x y \color{#D61F06}{y}=\sqrt{\color{#3D99F6}{x}\color{#D61F06}{y}}

Squring both sides.

y 2 = x y \color{#D61F06}{y}^2=\color{#3D99F6}{x}\color{#D61F06}{y}

y = x \color{#D61F06}{y}=\color{#3D99F6}{x}

x = x x x x x x x x x x x . . . \color{#3D99F6}{x}=\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x...}}}}}}}}}}}

This solution is incomplete for two reasons: First you do not show that the nested root actually converges, and secondly you need to examine the possibility that y = 0 y=0 but x 0 x \neq 0 .

Otto Bretscher - 5 years, 4 months ago

Interesting approach. One I had not thought of. Very clever, though.

Drex Beckman - 5 years, 4 months ago

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THANKS. \color{#3D99F6}{\text{THANKS.}}

A Former Brilliant Member - 5 years, 4 months ago
Jack Rawlin
Feb 4, 2016

n = x x x x x x x n = \sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\cdots}}}}}}}

n 2 = x x x x x x x x n^2 = x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\cdots}}}}}}}

n 2 = x n n^2 = xn

n 2 x n = 0 n^2 - xn = 0

n ( n x ) = 0 n(n - x) = 0

n = 0 , n x = 0 \therefore n = 0, \text{ } n - x = 0

n = 0 , x n = 0,\text{ } x

When n n is 0 0 :

0 = x x x x x x x 0 = \sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\cdots}}}}}}}

0 2 = x x x x x x x x 0^2 = x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\cdots}}}}}}}

x = 0 x = 0

x 0 x 0 x \geq 0 \therefore x \neq 0

Since x 0 x \neq 0 the only other solution is:

x = x x x x x x x x = \sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\cdots}}}}}}}

Cool solution, @Jack Rawlin. I hadn't thought of it that way, but it's very clear and logical.

Drex Beckman - 5 years, 4 months ago

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Thanks, I do try to make my solutions as logical as possible.

Jack Rawlin - 5 years, 4 months ago

How do we know that x != 0 from the inequality/work that you did? Thanks. @Jack Rawlin

Jason Guo - 5 years, 4 months ago
Drex Beckman
Feb 1, 2016

Notice this example: 4 16 = 4 16 4 = 4 \sqrt{4\sqrt{16}}=\sqrt{4}\cdot\sqrt[4]{16}=4 . So, as we add more roots, we divide by a higher power of 2. We can look at this as a geometric series: 1 1 2 n \sum_{1}^{\infty}\frac{1}{2^{n}} . And we can find the convergence of a geometric series with: a 1 r \frac{a}{1-\left | r \right |} . If we do that, we get 0.5 1 0.5 = 1 \frac{0.5}{1-0.5}=1 . So, lim x x x x x x x x x x x x . . . = x 1 \lim_{x\rightarrow \infty}\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x\sqrt{x...}}}}}}}}}}}=x^{1} .

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