Saturday, 9 July 2016

calculus - L'hopital's Rule on an indeterminate difference



I have the following question, in which I am told to use L'Hopital's rule:



$\lim_{x\to \infty}(-x+5\cdot ln(x))$



From eyeballing it, I would conclude that the polynomial $x$ will decrease faster than the logarithmic $ln$ would increase, meaning the limit would be $-\infty$, but I can't see how to use L'hopitals rule here as I've been told. I know that the idea is to put this into a quotient, but if I do the following:




$$-x+5\cdot ln(x) = \frac{-x^2}{x}+\frac{5x\cdot ln(x)}{x} = \frac{-x^2 + 5x\cdot ln(x)}{x}$$



And then try to take the limit, I get a doubly indeterminate form of $\frac{-\infty+\infty}{\infty}$, and using L'Hopitals rule here just ends back inthe same kind of indeterminate difference.


Answer



$$\begin{array}{lll}
\displaystyle\lim_{x\to \infty}(-x+5\ln x)&=&\displaystyle\lim_{x\to \infty}(-x+\ln x^5)\\
&=&\displaystyle\lim_{x\to \infty}(\ln(e^{-x+\ln x^5}))\\
&=&\ln\displaystyle\lim_{x\to \infty}(e^{-x+\ln x^5})\\
&=&\ln\displaystyle\lim_{x\to \infty}(e^{-x}e^{\ln x^5})\\
&=&\ln\displaystyle\lim_{x\to \infty}(x^5e^{-x})\\

&=&\ln\displaystyle\lim_{x\to \infty}\bigg(\frac{x^5}{e^x}\bigg)\\
&=&\dots\\
\end{array}$$


No comments:

Post a Comment

real analysis - How to find $lim_{hrightarrow 0}frac{sin(ha)}{h}$

How to find $\lim_{h\rightarrow 0}\frac{\sin(ha)}{h}$ without lhopital rule? I know when I use lhopital I easy get $$ \lim_{h\rightarrow 0}...