Saturday, 14 March 2015

integration - Evaluate limntoinftynint21fracdxx2(1+xn)



Evaluate lim




without Taylor expansion.



I tried rewriting as



\lim_{n\to \infty} n\int _1^2 \frac{1 + x^n - x^n}{x^2(1+x^n)}dx = \lim_{n\to \infty} n\int _1^2 \frac{dx}{x^2} - \lim_{n\to \infty} n\int _1^2 \frac{x^n}{x^2(1+x^n)}dx



The first integral is computable, but I don't know how to continue solving the second one.



The answer is \ln 2



Answer



By setting x=z^{1/n} we are left with
\lim_{n\to +\infty}\int_{1}^{2^n}\frac{dz}{z(1+z)z^{1/n}}=\lim_{n\to +\infty}\left[O\left(\frac{1}{2^n}\right)+\int_{1}^{+\infty}\left(\frac{1}{z}-\frac{1}{z+1}\right)\frac{dz}{z^{1/n}}\right]
then by applying the dominated convergence theorem we get that the answer is \color{red}{\log 2}.


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