How to find the limit $\lim_{x\to\infty}\frac{\ln(x^2+x+5)}{\ln(x^8-x+3)}$? I don't know how to approach this problem:
$$\lim_{x\to\infty}\frac{\ln(x^2+x+5)}{\ln(x^8-x+3)}$$
I tried reducing it to the form $\lim (1+x_n)^{1/x_n}=1$, but that didn't work.  

Here's a step by step solution:
Since both, numerator and denominator of
$$\frac{\ln(x^2+x+5)}{\ln(x^8-x+3)}$$ tend to $\infty$ when $x \rightarrow \infty$, we can apply L'Hôpital's rule

\begin{align*}
\lim_{x\rightarrow\infty}\frac{\ln(x^2+x+5)}{\ln(x^8-x+3)}&=\lim_{x\rightarrow\infty}\frac{\frac{d}{dx}\ln(x^2+x+5)}{\frac{d}{dx}\ln(x^8-x+3)}\\
&=\lim_{x\rightarrow\infty}\frac{\frac{2x+1}{x^2+x+5}}{\frac{8x^7-1}{x^8-x+3}}\\
&=\lim_{x\rightarrow\infty}\frac{(2x+1)(x^8-x+3)}{(x^2+x+5)(8x^7-1)}\\
&=\lim_{x\rightarrow\infty}\frac{2x^9+x^8-2x^2+5x+3}{8x^9+8x^8+40x^7-x^2-x-5}\\
&=\lim_{x\rightarrow\infty}\frac{2+\frac{1}{x}-\frac{2}{x^7}+\frac{5}{x^8}+\frac{3}{x^9}}{8+\frac{8}{x}+\frac{40}{x^2}-\frac{1}{x^7}-\frac{1}{x^8}-\frac{5}{x^9}}\\
&=\frac{1}{4}
\end{align*}
Note: Observe, that in the calculation above only the terms with the highest power of $x$ are relevant for the result.

