Example: Evaluate \(\displaystyle \lim_{x\to-\infty}\frac{x+3}{\sqrt{9x^2-5x}}\).
Solution:
As $x\to-\infty$, the leading term at the numerator is $x$. At the denominator, on the other hand, $(9x^2-5x)\sim 9x^2$, so the denominator approaches $\sqrt{9x^2}=3|x|$. Since $x\to-\infty$, it's safe to assume that $x<0$, so that $|x|=-x$. All together,
$$
\frac{x+3}{\sqrt{9x^2-5x}}\sim\frac{x}{-3x}\to-\frac13,
$$
which is the value of the limit. In partticular, this shows that $y=-1/3$ is a horizontal asymptote.
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