Saturday, August 11, 2018

calculus - Limit of xln(x) as x approaches +infty



To evaluate the limit of an even larger expression



lim




I need to evaluate part of the denominator to determine whether I could apply L'Hôpital's Rule



\lim_{x \to +\infty} x - \ln(x)



The problem is that I can't seem to manipulate the expression to the indeterminate forms 0/0 or \infty/\infty. I was thinking of multiplying by x/x



\lim_{x \to +\infty} \frac{x^2 - x\ln(x)}{x}



But then I get another indeterminate form \infty - \infty in the numerator. I was also thinking that x grows much faster than \ln x, so the limit obviously tends to +\infty but I don't think that would fly with most people :)


Answer



x-\ln{x}=\ln{e^x}-\ln{x}=\ln{\frac{e^x}{x}}\,, and \frac{e^x}{x} is in the form you want.


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