$(1)$
\[
[a] \
\frac{\partial p}{\partial x}dx
\]
$(2)$
\[
[b] \
\pi r^2
\hspace{20px}
[c]\
\pi r^2
\hspace{20px}
[d]\
\tau=-u\left(\frac{du}{dr}\right)
\]
$(3)$
\[
\tau=\frac{1}{2}\left(-\frac{dp}{dx}\right)r
\]
$(4)$
\[
\tau=-u\left(\frac{du}{dr}\right)
\]
$(5)$
\[
\frac{du}{dr}=-\frac{1}{2\mu}\left(-\frac{dp}{dx}\right)r
\]
$(6)$
\[\begin{align}
u=\int\frac{du}{dr}dr
&=\int\left\{-\frac{1}{2\mu}\left(-\frac{dp}{dx}\right)r\right\}dr\\
&=-\frac{1}{4\mu}\left(-\frac{dp}{dx}\right)r^2+C
\end{align}\]
$(7)$
\[
r=r_0においてu=0
\]
$(8)$
\[
u=\frac{1}{4\mu}\left(-\frac{dp}{dx}\right)\left({r_0}^2-r^2\right)
\]
$(9)$
\[
r=0\]
\[
u_{max}=\frac{1}{4\mu}\left(-\frac{dp}{dx}\right){r_0}^2
\]
$(10)$
\[
(\rm{I})\ 2\pi r
\hspace{20px}
(\rm{II})\ r_0
\hspace{20px}
(\rm{III})\ 8\mu
\hspace{20px}
(\rm{IV})\ 4
\]