Ik neem aan dat de rotatie uitschrijven lukt en dat je dus volgt tot en met:
\(\frac{{\partial p}}{{\partial z}}\left( {\frac{{\partial z}}{{\partial u}}\frac{{\partial x}}{{\partial v}} - \frac{{\partial x}}{{\partial u}}\frac{{\partial z}}{{\partial v}}} \right) + \frac{{\partial p}}{{\partial y}}\left( {\frac{{\partial y}}{{\partial u}}\frac{{\partial x}}{{\partial v}} - \frac{{\partial x}}{{\partial u}}\frac{{\partial y}}{{\partial v}}} \right)\)
Ik werk de haakjes uit:
\(\frac{{\partial p}}{{\partial z}}\frac{{\partial z}}{{\partial u}}\frac{{\partial x}}{{\partial v}} - \frac{{\partial p}}{{\partial z}}\frac{{\partial x}}{{\partial u}}\frac{{\partial z}}{{\partial v}} + \frac{{\partial p}}{{\partial y}}\frac{{\partial y}}{{\partial u}}\frac{{\partial x}}{{\partial v}} - \frac{{\partial p}}{{\partial y}}\frac{{\partial x}}{{\partial u}}\frac{{\partial y}}{{\partial v}}\)
Bij de twee positieve termen breng ik
\(\frac{{\partial x}}{{\partial v}}\) buiten, bij de negatieve termen
\(\frac{{\partial x}}{{\partial u}}\):
\(\left( {\frac{{\partial p}}{{\partial z}}\frac{{\partial z}}{{\partial u}} + \frac{{\partial p}}{{\partial y}}\frac{{\partial y}}{{\partial u}}} \right)\frac{{\partial x}}{{\partial v}} - \left( {\frac{{\partial p}}{{\partial z}}\frac{{\partial z}}{{\partial v}} + \frac{{\partial p}}{{\partial y}}\frac{{\partial y}}{{\partial v}}} \right)\frac{{\partial x}}{{\partial u}}\)
De truc (die ze impliciet toepassen) zit nu in het feit dat
\(\frac{{\partial p}}{{\partial x}}=0\) zodat je zonder probleem de volgende termen kan toevoegen, telkens de eerste term binnen de haakjes:
\(\left( {\frac{{\partial p}}{{\partial x}}\frac{{\partial x}}{{\partial u}} + \frac{{\partial p}}{{\partial y}}\frac{{\partial y}}{{\partial u}} + \frac{{\partial p}}{{\partial z}}\frac{{\partial z}}{{\partial u}}} \right)\frac{{\partial x}}{{\partial v}} - \left( {\frac{{\partial p}}{{\partial x}}\frac{{\partial x}}{{\partial v}} + \frac{{\partial p}}{{\partial y}}\frac{{\partial y}}{{\partial v}} + \frac{{\partial p}}{{\partial z}}\frac{{\partial z}}{{\partial v}}} \right)\frac{{\partial x}}{{\partial u}}\)
Maar wat er nu binnen de haakjes staat is gewoon de partiële afgeleide van p naar respectievelijk u en v, dus:
\(\frac{{\partial p}}{{\partial u}}\frac{{\partial x}}{{\partial v}} - \frac{{\partial p}}{{\partial v}}\frac{{\partial x}}{{\partial u}}\)