I hope my question is not too ridiculous, but why can differentiate a physical equation in the following manner:
$$\rho VA=\textrm{constant}$$
$$\frac{d\rho}{\rho}+\frac{dA}{A}+\frac{dV}{V}=0$$
It is the equation of continuity at steady state, and I understand that all the members are then divided by $\rho VA$ but why can we cancel $dt$ as if it was a constant?