If \(z = z(x , y)\), we can go through the motions of calculating \( \frac{\partial z}{\partial x}\) and \( \frac{\partial z}{\partial y}\), and we can then further calculate the second derivatives \(...If \(z = z(x , y)\), we can go through the motions of calculating \( \frac{\partial z}{\partial x}\) and \( \frac{\partial z}{\partial y}\), and we can then further calculate the second derivatives \( \frac{\partial ^2 z}{\partial x^2}\), \(\frac{\partial ^2 x}{\partial y^2}\), \( \frac{\partial ^2 z}{\partial y \partial x}\) and \(\frac{\partial ^2 z}{\partial y \partial x}\).