Solutions to be posted as a "Reply" only after January 22, 21:00

**a.** Find the general solution of

\begin{equation}

xu_x+4 yu_y=0

\label{eq-HA1.2}

\end{equation}

in $ \{(x,y)\ne (0,0)\}$; when this solution is continuous at $(0,0)$?

**b.** Find the general solution of

\begin{equation}

xu_x-4yu_y=0

\label{eq-HA1.3}

\end{equation}

in $ \{(x,y)\ne (0,0)\}$; when this solution is continuous at $(0,0)$?

**c.** Explain the difference between (\ref{eq-HA1.2}) and (\ref{eq-HA1.3}).