Try to rationalize this!

Calculus Level 3

Evaluate lim x + ( x + 1 ) 2015 + 2013 x 2000 2015 x \displaystyle \lim_{x \rightarrow + \infty } \sqrt[2015]{\left(x+1 \right)^{2015} + 2013x^{2000}} -x


The answer is 1.

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1 solution

Substituting x = 1 / h x = 1/h . we have h 0 + h \to 0+ as x x \to \infty .

So rewriting the expression we have

( ( 1 + h ) 2015 h 2015 + 2013 h 2000 ) 1 2015 1 h (\frac{(1+h)^{2015}}{h^{2015}} + \frac{2013}{h^{2000}})^{\frac{1}{2015}} - \frac{1}{h}

so we have:-

l i m h 0 + lim_{h\to0^{+}} ( ( 1 + h ) 2015 + 2013 h 5 ) 1 2015 1 h \frac{((1+h)^{2015} + 2013h^{5})^{\frac{1}{2015}} - 1}{h} which is ( 0 0 ) (\frac{0}{0}) form

So By L'Hospital Rule:-

P.S:- We can also make an approximation at this step as ( ( 1 + h ) 2015 + 2013 h 5 ) 1 2015 ((1+h)^{2015} + 2013h^{5})^{\frac{1}{2015}} tends to 1 + h 1+h as h 0 + h \to 0^{+}

we have our answer as 1 1 .

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