Thursday, December 31, 2015

real analysis - Use L'Hopital's rule to show that $lim_{x rightarrow+infty} frac{f(x)}{g(x)}= ell$

Let $f : \mathbb{R} \rightarrow \mathbb{R}$, $g : \mathbb{R} \rightarrow \mathbb{R}$, be both differentiable. Suppose that $\lim_{x \rightarrow + \infty} f(x) = \lim_{x \rightarrow + \infty} g(x) = 0$, that $g'(x) ≠ 0$ for all $x \in \mathbb{R}$ and $\lim_{x \rightarrow +\infty} \frac{f'(x)}{g'(x)} = \ell \in \mathbb{R}$. Show that



$$\lim_{x \rightarrow+\infty} \frac{f(x)}{g(x)}= \ell$$



I'm immediately thinking L'Hopital's rule, and investigating when x tends to an element of $\mathbb{R}$. I just learnt this however, how would I go forth to use this (assuming I do actually need to use L'Hopital's rule)?

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