/sci/ - Science & Math

I don't get this part
why does [math]\frac{\partial}{\partial t}\frac{\partial z}{\partial x}=\frac{\partial ^2z}{\partial x^2}\frac{\partial x}{\partial t}+\frac{\partial ^2z}{\partial x\partial y}\frac{\partial y}{\partial t}[/math]

I really don't get why the chain rule was used here

>>9579077
forgot image