Help me please.

1. Solve the equation:

$\displaystyle x_{2}\frac{\partial u}{\partial x_{1}}-x_{1}\frac{\partial u}{\partial x_{2}}=f$

What can be a noncharacteristic initial curve?

2. Solve:

$\displaystyle u\frac{\partial u}{\partial x_{1}}+\frac{\partial u}{\partial x_{2}}=1$

$\displaystyle u=\frac{s}{2} , x_{1}=x_{2}=s, 0<s<1$