Consider the equations

$\frac{\text{d}\text{e}\text{l}\text{u}}{\text{d}\text{e}\text{l}\text{x}}+\frac{\text{d}\text{e}\text{l}\text{v}}{\text{d}\text{e}\text{l}\text{y}}=0,\frac{\text{d}\text{e}\text{l}\text{u}}{\text{d}\text{e}\text{l}\text{y}}-\frac{\text{d}\text{e}\text{l}\text{v}}{\text{d}\text{e}\text{l}\text{x}}=0$

Show that this system is elliptic,

$($a$)$ directly,

$($b$)$ by introducing the variable $,$ where $u=$del$\frac{}{d}$elx and $v=$del$\frac{}{d}$elv.