Simple power limit

Calculus Level 3

lim x ( x x 2014 + x 2013 2014 ) \large \lim_{x\to \infty} \left ( x - \sqrt[2014]{x^{2014} + x^{2013}} \right)

If the limit above can be expressed as a b -\frac ab for coprime positive integers a a and b b , find the value of a + b a+b .


The answer is 2015.

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3 solutions

L = lim x ( x ( x 2014 ) ( 1 + 1 x ) 2014 ) L = \displaystyle \lim_{x \to \infty} \left(x - \sqrt[2014]{(x^{2014})(1+\dfrac{1}{x})}\right)

L = lim x ( x x ( 1 + 1 x 2014 ) ) \Rightarrow L = \displaystyle \lim_{x \to \infty} \left(x - x \left(\sqrt[2014]{1 + \dfrac{1}{x}} \right) \right)

Now as x , 1 x 0 x \to \infty, \dfrac{1}{x} \to 0

lim x ( 1 + 1 x ) 1 2014 = 1 + 1 2014 x + O ( 1 x 2 ) \Rightarrow \displaystyle \lim_{x \to \infty} \left(1 + \dfrac{1}{x}\right)^{\frac{1}{2014}} = 1 + \dfrac{1}{2014x} + O\left(\dfrac{1}{x^2}\right)

L = lim x ( x x ( 1 + 1 2014 x ) ) = 1 2014 \Rightarrow L = \displaystyle \lim_{x \to \infty} \left(x - x\left(1 + \dfrac{1}{2014x} \right)\right) = \boxed{-\dfrac{1}{2014}}

Done it the same way. :) . You should mention that:

lim x ( 1 + 1 x ) 1 2014 = 1 + 1 2014 x + O ( 1 x 2 ) \lim_{x \to \infty} \left ( 1 + \frac1x \right)^\frac1{2014} = 1 + \frac1{2014x} + O \left (\frac1{x^2}\right)

Satyajit Mohanty - 5 years, 10 months ago
Kenny Lau
Jul 24, 2015

We have: lim x ( x x 2014 + x 2013 2014 ) \lim_{x\to\infty}\left({x-\sqrt[2014]{x^{2014}+x^{2013}}}\right) Take h = 1 x 0 h=\frac1x\to0 : = lim h 0 ( 1 h 1 h 2014 + 1 h 2013 2014 ) =\lim_{h\to0}\left({\frac1h-\sqrt[2014]{\frac1{h^{2014}}+\frac1{h^{2013}}}}\right) Take common factor: = lim h 0 ( 1 h 1 h 2014 2014 1 + h 2014 ) =\lim_{h\to0}\left({\frac1h-\sqrt[2014]{\frac1{h^{2014}}}\sqrt[2014]{1+h}}\right) Simplify: = lim h 0 ( 1 h 1 h 1 + h 2014 ) =\lim_{h\to0}\left({\frac1h-\frac1h\sqrt[2014]{1+h}}\right) Take common factor: = lim h 0 ( 1 h ( 1 1 + h 2014 ) ) =\lim_{h\to0}\left({\frac1h\left({1-\sqrt[2014]{1+h}}\right)}\right) = lim h 0 ( 1 h ( 1 2014 1 + h 2014 ) ) =\lim_{h\to0}\left({\frac1h\left({\sqrt[2014]1-\sqrt[2014]{1+h}}\right)}\right) Using lemma below: = lim h 0 ( 1 h × ( 1 ) ( 1 + h ) 1 2013 2014 + 1 2012 ( 1 + h ) 1 2014 + + ( 1 + h ) 2013 2014 ) =\lim_{h\to0}\left({\frac1h\times\frac{(1)-(1+h)}{\sqrt[2014]{1^{2013}}+\sqrt[2014]{1^{2012}(1+h)^1}+\cdots+\sqrt[2014]{(1+h)^{2013}}}}\right) Expanding numerator: = lim h 0 ( 1 h × h 1 2013 2014 + 1 2012 ( 1 + h ) 1 2014 + + ( 1 + h ) 2013 2014 ) =\lim_{h\to0}\left({\frac1h\times\frac{-h}{\sqrt[2014]{1^{2013}}+\sqrt[2014]{1^{2012}(1+h)^1}+\cdots+\sqrt[2014]{(1+h)^{2013}}}}\right) Cancelling: = lim h 0 1 1 2013 2014 + 1 2012 ( 1 + h ) 1 2014 + + ( 1 + h ) 2013 2014 =\lim_{h\to0}-\frac{1}{\sqrt[2014]{1^{2013}}+\sqrt[2014]{1^{2012}(1+h)^1}+\cdots+\sqrt[2014]{(1+h)^{2013}}} Substituting: = 1 1 + 1 + + 1 =-\frac{1}{1+1+\cdots+1}

= 1 2014 =-\frac{1}{2014}

Lemma: x y = x n y n i = 0 n 1 x n i 1 y i x-y=\frac{x^n-y^n}{\sum_{i=0}^{n-1}x^{n-i-1}y^i}

Moderator note:

There's a much shorter approach. Does this look familiar to you?

= lim h 0 ( 1 h ( 1 2014 1 + h 2014 ) ) \displaystyle =\lim_{h\to0}\left({\frac1h\left({\sqrt[2014]1-\sqrt[2014]{1+h}}\right)}\right)

Hint : Definition of derivative.

Alisa Meier
Jul 23, 2015

lim x ( x x 2014 x 2013 2014 ) = \lim_{x \to \infty} (x - \sqrt[2014]{x^{2014} - x^{2013}}) =

lim x x ( 1 ( 1 + 1 x ) 1 2014 ) = \lim_{x \to \infty} x*(1 - (1+{1\over x})^ {1 \over 2014} )=

lim x x ( 1 1 ( ( 1 + 1 x ) 2014 ) 1 ) = \lim_{x \to \infty} x*(1-{{1 \over ({(1+ {1 \over x}})^{2014})}}^{-1} )=

lim x x ( 1 1 ( 1 + 1 x + 2014 1 x + ( 2014 2 ) 1 x + ) 1 ) = 1 2014 \lim_{x \to \infty} x*(1-{{1 \over {(1+ {1 \over x} + 2014 * {1 \over x} + \binom{2014}{2} * {1 \over x } + \dots }})}^{-1} )= {-1 \over 2014}

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