Answer
a)
$$
\begin{aligned}
\lim _{R \rightarrow r^{+}} v &=\lim _{R \rightarrow r^{+}}\left[-c\left(\frac{r}{R}\right)^{2} \ln \left(\frac{r}{R}\right)\right] \\
&=-c r^{2} \lim _{R \rightarrow n^{+}}\left[\left(\frac{1}{R}\right)^{2} \ln \left(\frac{r}{R}\right)\right] \\
&=-c r^{2} \cdot \frac{1}{r^{2}} \cdot \ln 1\\
&=-c \cdot 0\\
&=0
\end{aligned}
$$
As the insulation of a metal cable becomes thinner, the velocity of an electrical impulse in the cable approaches zero.
b)
$$
\begin{aligned}
\lim _{r \rightarrow 0^{+}} v &=\lim _{r \rightarrow 0^{+}}\left[-c\left(\frac{r}{R}\right)^{2} \ln \left(\frac{r}{R}\right)\right]\\
&=-\frac{c}{R^{2}} \lim _{r \rightarrow 0^{+}}\left[r^{2} \ln \left(\frac{r}{R}\right)\right]\\
& \,\,\,\,\,[ \text{form is} \,\, 0 · \infty]\\
&=-\frac{c}{R^{2}} \lim _{r \rightarrow 0^{+}} \frac{\ln \left(\frac{r}{R}\right)}{\frac{1}{r^{2}}}\\
& \,\,\,\,\,[ \text{form is} \,\, \infty /\infty \,\, \text {and by using L'Hôpital's rule we have} ]\\
&=-\frac{c}{R^{2}} \lim _{r \rightarrow 0^{+}} \frac{\frac{R}{r} \cdot \frac{1}{R}}{\frac{-2}{r^{2}}}\\
&=-\frac{c}{R^{2}} \lim _{r \rightarrow 0^{+}}\left(-\frac{r^{2}}{2}\right)\\
&=0
\end{aligned}
$$
As the radius of the metal cable approaches zero, the velocity of an electrical impulse in the cable approaches zero.
Work Step by Step
a)
$$
\begin{aligned}
\lim _{R \rightarrow r^{+}} v &=\lim _{R \rightarrow r^{+}}\left[-c\left(\frac{r}{R}\right)^{2} \ln \left(\frac{r}{R}\right)\right] \\
&=-c r^{2} \lim _{R \rightarrow n^{+}}\left[\left(\frac{1}{R}\right)^{2} \ln \left(\frac{r}{R}\right)\right] \\
&=-c r^{2} \cdot \frac{1}{r^{2}} \cdot \ln 1\\
&=-c \cdot 0\\
&=0
\end{aligned}
$$
As the insulation of a metal cable becomes thinner, the velocity of an electrical impulse in the cable approaches zero.
b)
$$
\begin{aligned}
\lim _{r \rightarrow 0^{+}} v &=\lim _{r \rightarrow 0^{+}}\left[-c\left(\frac{r}{R}\right)^{2} \ln \left(\frac{r}{R}\right)\right]\\
&=-\frac{c}{R^{2}} \lim _{r \rightarrow 0^{+}}\left[r^{2} \ln \left(\frac{r}{R}\right)\right]\\
& \,\,\,\,\,[ \text{form is} \,\, 0 · \infty]\\
&=-\frac{c}{R^{2}} \lim _{r \rightarrow 0^{+}} \frac{\ln \left(\frac{r}{R}\right)}{\frac{1}{r^{2}}}\\
& \,\,\,\,\,[ \text{form is} \,\, \infty /\infty \,\, \text {and by using L'Hôpital's rule we have} ]\\
&=-\frac{c}{R^{2}} \lim _{r \rightarrow 0^{+}} \frac{\frac{R}{r} \cdot \frac{1}{R}}{\frac{-2}{r^{2}}}\\
&=-\frac{c}{R^{2}} \lim _{r \rightarrow 0^{+}}\left(-\frac{r^{2}}{2}\right)\\
&=0
\end{aligned}
$$
As the radius of the metal cable approaches zero, the velocity of an electrical impulse in the cable approaches zero.