Root 2014?!

Level pending

If

x = 1 + 2 2014 + 4 2014 + 8 2014 + + 2 2013 2014 x=1+\sqrt[2014]{2}+\sqrt[2014]{4}+\sqrt[2014]{8}+\dots+\sqrt[2014]{2^{2013}}

Then, the value of ( x + 1 x ) 95 (\frac{x+1}{x})^{95} can be expressed in the form 2 a b \sqrt[b]{2^a} , where a a and b b are positive coprime integers. What is the value of a + b a+b ?


The answer is 111.

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2 solutions

Josh Petrin
Jan 2, 2014

Let q = 2 2014 q = \sqrt[2014]{2} . Then x = q 2013 + q 2012 + q 2011 + + q + 1 = q 2014 1 q 1 = 1 2 2014 1 . \begin{aligned} x &= q^{2013} + q^{2012} + q^{2011} + \dots + q + 1 \\ &= \frac{q^{2014} - 1}{q - 1} \\ &= \frac{1}{\sqrt[2014]{2} - 1}. \end{aligned} We have therefore ( x + 1 x ) 95 = ( 1 + 1 x ) 95 = ( 1 + 2 2014 1 ) 95 = 2 95 2014 . \begin{aligned} \left( \frac{x+1}{x} \right)^{95} &= \left( 1 + \frac{1}{x} \right)^{95} \\ &= (1 + \sqrt[2014]{2} - 1)^{95} \\ &= \sqrt[2014]{2^{95}}. \end{aligned} And, finally, since 2014 = 19 106 2014 = 19 \cdot 106 and 95 = 19 5 95 = 19 \cdot 5 , 2 95 2014 = 2 5 106 , \sqrt[2014]{2^{95}} = \sqrt[106]{2^{5}}, and a + b = 111 a + b = \boxed{111} .

Matthew Fan
Jan 1, 2014

One should not simply borrow questions from SMO.

Anyway, multiply the equation by 2 2014 \sqrt[2014]{2} .

Now, taking this and subtracting the original, we get ( 2 2014 1 ) x = 1 (\sqrt[2014]{2}-1)x=1 . So now 1 + 1 x = 2 2014 1+\frac{1}{x}=\sqrt[2014]{2} and hence, we get our conclusion of 111

Yea sorry I didn't state that it is adapted from SMO :/

Ryan Phua - 7 years, 5 months ago

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