How is mass flow rate within an annular region of a pipe taken to be an inexact differential? I read it in Fluid Mechanics textbook by Yunus A. Cengel and John M. Cimbala. The mass flow rate through the annulus is given to be inexact differential. Why is mass flow through the annulus not equal to (m2)-(m1) Given any 2 radius r2 and r1? Won't the mass flow rate in the annulus be equal to (mass flow through the area with radius r2)-(mass flow through the area with radius r1) ie m2-m1? Also later it goes on to say that the mass flow rate is exact. Thank you.