Root of powers

Algebra Level 1

2 2 = ? \large \sqrt{ 2^{ \sqrt{2} } } = \, ?

( 2 ) 2 4 \big(\sqrt{2}\big)^{\sqrt[4]{2}} 2 2 4 2^{\sqrt[4]{2}} ( 2 ) 2 \big(\sqrt{2}\big)^{\sqrt{2}} 2 2

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8 solutions

2 2 = ( 2 2 ) 1 / 2 = ( 2 1 / 2 ) 2 = 2 2 \Large \sqrt{2^{\sqrt{2}}} = (2^{\sqrt{2}})^{1/2} = (2^{1/2})^{\sqrt{2}} = \boxed{\sqrt{2}^{\sqrt{2}}} .

Moderator note:

Please note: the specific rule is

( a b ) c = ( a c ) b (a^b)^c = (a^c)^b

not

a ( b c ) = a ( c b ) a^{(b^c)} = a^{(c^b)}

or some idea that in general you can simply change the order exponents are taken. For example:

2 ( 3 4 ) = 2417851639229258349412352 2^{(3^4)} = 2417851639229258349412352 and 2 ( 4 3 ) = 18446744073709551616 2^{(4^3)} = 18446744073709551616

but on the other hand

( 2 3 ) 4 = 4096 (2^3)^4 = 4096 and ( 2 4 ) 3 = 4096. (2^4)^3 = 4096 .

Did the same

Peter van der Linden - 3 years, 4 months ago

I was surprised that the root "went in directly", then I worked out the math like you did :)

Chung Kevin - 3 years, 4 months ago
Naren Bhandari
Feb 11, 2018

N = 2 2 N = ( 2 2 ) 1 2 N = 2 2 2 c c N = ( 2 1 2 ) 2 N = 2 2 \large\begin{aligned} & N = \sqrt{2^{ \sqrt{2}}} \\& N =\left(2 ^{\sqrt 2 }\right)^{\frac{1}{2}} \\& N =2^{\frac{\sqrt 2}{2}} \phantom{cc}\\& N = \left(2^{\frac{1}{2}}\right)^{\sqrt 2} \\& N = \sqrt{2}^{\sqrt2}\end{aligned}

Moderator note:

More generally,

a n m = ( a n ) 1 m = ( a n m ) = ( a 1 m ) n = a m n \sqrt[m]{ a^{\sqrt{n}} } = \left( a^{\sqrt{n}} \right)^{\frac{1}{m}} = \left( a^{\frac{ \sqrt{n}}{m}} \right) = \left( a^{\frac {1}{m}} \right)^{\sqrt{n}} = \sqrt[m]{a}^{\sqrt{n}}

Mohammad Khaza
Feb 12, 2018

2 2 \large\sqrt{2^{\sqrt2}}

= 2 1 / 2 2 \large{{2}^{{1/2}}}^{\sqrt{2}}

= 2 2 \large{\sqrt{2}}^{\sqrt{2}} ........................[ 2 1 / 2 = 2 2^{1/2}=\sqrt{2} ]

Jubaer Al-Amin
Feb 12, 2018

L e t a = 2 A n d x = 2 2 x = 2 a x 2 = 2 a x 2 = ( 2 ) 2 a x = 2 a x = 2 2 Let\ a = \sqrt{2} \\ And\ x = \sqrt{2^{\sqrt{2}}} \\x = \sqrt{2^a} \\x^2 = 2^a \\x^2 = (\sqrt{2})^{2a} \\x = \sqrt{2}^a\\x = \sqrt{2}^{\sqrt{2}}

Oh very creatively done. This is the simplest algebraic work to follow through. +1

Pi Han Goh - 3 years, 3 months ago
Martin Muqa
Feb 13, 2018

Look at the image below, to see the solution and/or proofs to this problem, that I got. It's a bit blurry, but you'll live to tell the tale:

Nicely written. By the way, you might be interested in this reading this common misconception .

Pi Han Goh - 3 years, 3 months ago

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Interesting... Thanks :)

Martin Muqa - 3 years, 3 months ago

Thank you. Easy to understand.

Tyra Redwood - 3 years, 3 months ago
Paul Sinnett
Feb 18, 2018

I played with this for a while but I couldn't get to any of the answers. So I guessed I must be missing an identity of 2 \sqrt{2} or something. That led me to Gelfond–Schneider constant , which just flat out states 2 2 = 2 2 \sqrt{2^{\sqrt{2}}} = \sqrt{2}^{\sqrt{2}} but doesn't explain it.

It is a general identity, not just of 2 \sqrt{2} . Brian and Naren's solution shows how the exponent "moves around".

Chung Kevin - 3 years, 3 months ago
Gaurav Astronaut
Feb 17, 2018

When i calculated the value of both there was slight difference in later digits after decimal. Can anyone explain why that happened?

It could be the rounding error when you used a calculator.

Chung Kevin - 3 years, 3 months ago
L M
Feb 15, 2018

I think 2 1 2 2^{\sqrt{\frac{1}{2}}} is more elegant

Yes, that's a nice way of writing it too. Unfortunately, that doesn't extend to the general case. I was thinking of:

a n m = ( a n ) 1 m = ( a n m ) = ( a 1 m ) n = a m n \sqrt[m]{ a^{\sqrt{n}} } = \left( a^{\sqrt{n}} \right)^{\frac{1}{m}} = \left( a^{\frac{ \sqrt{n}}{m}} \right) = \left( a^{\frac{1}{m}} \right)^{\sqrt{n}} = \sqrt[m]{a}^{\sqrt{n}}

Chung Kevin - 3 years, 3 months ago

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