1. The problem statement, all variables and given/known data In a shallow layer of water, the velocity of water in the z direction may be ignored and is therefore [tex](\dot{x},\dot{y})[/tex]. We can define the Lagrangian coordinates such that the depth of water h is satisfied by the relations Given that [tex]h = \frac{1}{\alpha}[/tex] and [tex]\alpha = \frac{\partial(x,y)}{\partial(a,b)}[/tex] and the Lagrangian density is given as [tex]L = \frac{1}{2}\dot{x}^2 + \frac{1}{2}\dot{y}^2 - \frac{1}{2}gh(x_a,x_b,y_a,y_b)[/tex] where [tex]p_q = \frac{\partial p}{\partial q}[/tex]. Given further that Lagrange's equations for a 2D continuous system are known to be [tex] \frac{D}{Dt}\left( \frac{\partial L}{\partial\dot{x}} \right) + \frac{\partial}{\partial a} \left( \frac{\partial L}{\partial x_a} \right) + \frac{\partial}{\partial b} \left( \frac{\partial L}{\partial x_b} \right) - \frac{\partial L}{\partial x} = 0 [/tex] with a similar equation for the y variable, prove that [tex] \frac{D\dot{x}}{Dt} + g \frac{\partial h}{\partial x} = 0 [/tex] 2. Relevant equations I know the general approach of this problem, but my main problem comes in substituting [tex] \frac{\partial}{\partial a} \frac{\partial L}{\partial x_a} [/tex]. If I apply chain rule on the Lagrangian here, [tex] \frac{\partial}{\partial a} \frac{\partial L}{\partial h} \frac{\partial h}{\partial x_a} = -\frac{g}{2} \frac{\partial}{\partial a} \frac{\partial h}{\partial x_a} [/tex] How do I proceed after this point?