I do not understand the concept of differential in physics for example
What have both the sides been differentiated by to get this result.I understand that $dz^2/dz$ gives $2z$, where has the extra $dz$ come from? Also, what has the side with $\theta$ been differentiated by, to get this result? It cannot be $dz$. If it's differentiated with respect to $\theta$, how can you differentiate something with respect to another thing in two sides of an equation? Can someone explain this and how did $dz$ and $d\theta$ come in the equations?