# Toronto Math Forum

## MAT334--2020F => MAT334--Tests and Quizzes => Quiz 2 => Topic started by: kaye on October 02, 2020, 01:06:57 PM

Title: Quiz 2 f(z)=(z-2)\log|z-2| at z_0=2
Post by: kaye on October 02, 2020, 01:06:57 PM
Problem(3pt). Find the limit of each function at the given point, or explain why it does not exist.
$$f(z)=(z-2)\log|z-2| \text{ at } z_0=2$$

Let $z'=z-2$, then
\begin{align*}
\lim_{z \to \infty} |f(z)| &=\lim_{z' \to 0} |f(z)|\\
&= \lim_{z' \to 0} |z'\log|z'||\\
&= \lim_{z' \to 0} \frac{\log|z'|}{\frac{1}{|z'|}}
\end{align*}
When $z' \to 0$, we get $\frac{\infty}{\infty}$, now use the L'Hospital's Rule we have:
\begin{align*}
\lim_{z \to \infty} |f(z)| &= \lim_{z' \to 0} \frac{\log|z'|}{\frac{1}{|z'|}}\\
&= \lim_{z' \to 0} \frac{\frac{1}{|z'|}}{\frac{-1}{|z'|^2}}\\
&= \lim_{z' \to 0} - |z'|\\
&= 0
\end{align*}
The limit of $f(z)$ at $z_0=2$ is 0.