Nested Rooted 2's

Algebra Level 3

( 2 ) ( 2 ) ( 2 ) . . . = ? \Large \left(\sqrt2 \right)^{{\left(\sqrt2 \right)}^{{\left(\sqrt2 \right)}^{.^{.^{.}}}} } = \ ?

This problem is a part of set nested radicals .
2 \sqrt{2} 1 2 impossible to find

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4 solutions

Hem Shailabh Sahu
Feb 15, 2015

Let ( 2 ) ( 2 ) = x (\sqrt2)^{{(\sqrt2)^{\ldots}}} = x

x = ( 2 ) x = 2 x 2 x = (\sqrt2)^x=2^{\frac{x}{2}}

x = 2 x 2 \Rightarrow x=2^{\frac{x}{2}}

Squaring Both Sides,

x 2 = 2 x x^2=2^x

Comparing both sides, x = 2 \boxed{x=2}

x x can also be 4 4 .

Francisco Rodríguez - 6 years, 3 months ago

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Doesn't fit the given options :)

Hem Shailabh Sahu - 6 years, 3 months ago

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Can you explain, please, me why 4 is not a solution? I can't get it.

Andrea Palma - 6 years, 2 months ago

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@Andrea Palma No one said it wasn't a possible answer, they just pointed out that it wasn't one of the multiple choice answers, so it didn't matter.

Louis W - 6 years ago

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@Louis W I think they did say it wasn't a possible answer. If x = 2 2 2 . . . x=\sqrt{2}^{\sqrt{2}^{\sqrt{2}^{...}}} , x = 2 x=2 , and x = 4 x=4 , then 2 = 4 2=4 ?

The only thing I can guess is when squaring both sides, an 'extraneous solution' was added, but I wouldn't know how to check for extraneous solutions.

I used a different method which I don't think has alternate solutions. I will post it shortly.

Chad Lower - 5 years, 12 months ago

misalkan V2^V2^V2^... =x (V2)^x=x 2^x = x^2 so,x=2

Chad Lower
Jun 17, 2015

Let x = ( 2 ) ( 2 ) ( 2 ) x=(\sqrt{2})^{(\sqrt{2})^{(\sqrt{2})^{\ldots}}} . Then x = ( 2 ) ( 2 2 ) ( 2 ) x=(2)^{\left(\frac{\sqrt{2}}{2}\right)^{(\sqrt{2})^{\ldots}}} x = ( 2 ) ( 1 2 ) ( 2 ) x=(2)^{\left(\frac{1}{\sqrt{2}}\right)^{(\sqrt{2})^{\ldots}}} x = ( 2 ) ( 1 ( 2 ) 2 ( 2 ) ) x=(2)^{\left(\frac{1^{\left(\sqrt{2}\right)^{\ldots}}}{\sqrt{2}^{\left(\sqrt{2}\right)^{\ldots}}}\right)} x = ( 2 ) ( 1 x ) x=(2)^{\left(\frac{1}{x}\right)} Take the log of both sides gives ln x = ln ( 2 ) ( 1 x ) \ln{x}=\ln{(2)^{\left(\frac{1}{x}\right)}} ln x = ( 1 x ) ln ( 2 ) \ln{x}={\left(\frac{1}{x}\right)}\ln{(2)} x ln x = ln ( 2 ) x\ln{x}=\ln{(2)} ln x x = ln ( 2 ) \ln{x^{x}}=\ln{(2)} Raise both sides as the powers of common base e e to get x x = ( 2 ) x^{x}=(2) But note that due to the infinite nature of the exponents, x x = ( 2 ) ( 2 ) ( 2 ) ( 2 ) ( 2 ) ( 2 ) = x x^{x}=(\sqrt{2})^{(\sqrt{2})^{(\sqrt{2})^{(\sqrt{2})^{(\sqrt{2})^{(\sqrt{2})^{\ldots}}}}}}=x So by the transitive property, x x = x = 2 x^{x}=x=2

Deepesh Verma
Feb 27, 2015

Let \sqrt{2}^{x}= 2^{x/2} Log 2{x}= x/2 log 2{2} 2 log 2{x}=xlog 2{2} Comparing both the side... x=2

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