nx = xⁿ

Algebra Level 2

We have a simple equation; it's n x = x n nx = x^n . n n has to be a natural number, x x can be an irrational number. To satisfy the equation n x = x n nx = x^n for n = 2017 n = 2017 , what can x |{x}| be, else than 0?

2017/2016 Nothing (1/2017)^2017 2017^(1/2017) 2017/2018 2017^(1/2016) 2016^(1/2017)

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2 solutions

Casper de With
Oct 21, 2017

The solution is 2017^(1/2016) ( or 2017 2016 \sqrt[2016]{2017} ). Why? Let's solve this as an equality.

2017 x = x 2017 2017x = x^{2017}

Divide both sides by x x ; left side becomes 2017 2017 , right side loses one power of x x :

2017 = x 2016 2017 = x^{2016}

Take the 2016th square root of both sides; left side becomes 2017 2016 \sqrt[2016]{2017} , right side becomes x x (because x 2016 2016 = x \sqrt[2016]{x^{2016}} = x ):

2017 2016 = x \sqrt[2016]{2017} = x

Et voilà, you've found x x ; it's 2017 2016 \sqrt[2016]{2017} .

Note: x x can also be 0 0 or 2017 2016 -\sqrt[2016]{2017} because n n is odd, but the answer was asked to be the absolute value of x x ( x |x| ) and not being equal to 0 0 .

Chew-Seong Cheong
Oct 25, 2017

n x = x n n = x n 1 log n = ( n 1 ) log x log x = log n n 1 = log n 1 n 1 x = n 1 n 1 = 201 7 1 2016 \begin{aligned} nx&=x^n \\ n &=x^{n-1} \\ \log n &= (n-1) \log x \\ \log x &= \frac {\log n}{n-1} = \log n^{\frac 1{n-1}} \\ \implies x &= n^{\frac 1{n-1}} = \boxed{2017^{\frac 1{2016}}} \end{aligned}

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