This is LaTeX code:

[latex]
\frac {1} {\mu}\cdot \mu '= \frac 
{\frac {\partial f_1} {\partial y} - \frac {\partial f_2} {\partial x}} 
{\frac {\partial v} {\partial x}\cdot f_2 - \frac {\partial v} {\partial y}\cdot f_1}
[/latex]