$\renewcommand{\Re}{\operatorname{Re}}$ $\renewcommand{\Im}{\operatorname{Im}}$ $\newcommand{\erf}{\operatorname{erf}}$ $\newcommand{\dag}{\dagger}$ $\newcommand{\const}{\mathrm{const}}$
Problem 1. Consider first order equations and determine if they are linear homogeneous, linear inhomogeneous, or non-linear ($u$ is an unknown function) \begin{gather} u_t+xu_x= 0,\\[2pt] u_t+uu_x= 0,\\[2pt] u_t+xu_x- u=0,\\[2pt] u_t+u u_x+x=0,\\[2pt] u_t + u_x -u^2=0,\\[2pt] u_t^2-u_x^2-1=0,\\[2pt] u_x^2+u_y^2-1=0,\\[2pt] x u_x + y u_y+ zu_z=0,\\[2pt] u_x^2 + u_y^2+ u_z^2-1=0,\\[2pt] u_t + u_x^2+u_y^2=0. \end{gather} For non-linear equations determine if they are quasilinear (quasilinear= linear with respect to first-order derivatives $(u_x,u_y)$, but not to derivatives and function itself $(u_x,u_y,u)$.
Problem 2. Consider equations and determine their order; determine if they are linear homogeneous, linear inhomogeneous or non-linear ($u$ is an unknown function): \begin{gather} u_t+ (1+x^2)u_{xx}=0,\\[2pt] u_t- (1+u^2)u_{xx}=0,\\[2pt] u_t +u_{xxx}=0,\\[2pt] u_t +uu_x+u_{xxx}=0,\\[2pt] u_{tt}+u_{xxxx}=0,\\[2pt] u_{tt}+u_{xxxx}+u=0,\\[2pt] u_{tt}+u_{xxxx}+\sin(x)=0,\\[2pt] u_{tt}+u_{xxxx}+\sin(x)\sin(u)=0. \end{gather}
**Problem 3. Find the general solutions to the following equations \begin{gather} u_{xy}=0,\\[2pt] u_{xy}= 2u_x,\\[2pt] u_{xy}=e^{x+y},\\[2pt] u_{xy}= 2u_x+e^{x+y}. \end{gather} Hint: Introduce $v=u_x$ and find it first.
Problem 4. Find the general solutions to the following equations \begin{gather} u u_{xy}=u_xu_y,\\[2pt] u u_{xy}= 2u_xu_y,\\[2pt] u_{xy}=u_x u_y \end{gather} Hint: Divide two first equations by $uu_x$ and observe that both the right and left-hand expressions are derivative with respect to $y$ of $\ln (u_x)$ and $\ln (u)$ respectively. Divide the last equation by $u_x$.
Problem 5. Find the general solutions to the following equations \begin{gather} u_{xxyy}=0, \\[2pt] u_{xyz}= 0,\\[2pt] u_{xxyy}=\sin(x)\sin(y),\\[2pt] u_{xyz}= \sin(x)\sin(y)\sin(z),\\[2pt] u_{xyz}= \sin(x)+\sin(y)+\sin(z). \end{gather}