Formula_5333

\[\Delta u \equiv \frac{\partial}{\partial \xi}\left(\frac{F \frac{\partial u}{\partial \eta}-G \frac{\partial u}{\partial \xi}}{\sqrt{E G-F^{2}}}\right)+\frac{\partial}{\partial \eta}\left(\frac{F \frac{\partial u}{\partial \xi}-E \frac{\partial u}{\partial \eta}}{\sqrt{E G-F^{2}}}\right)=0 .\]