The reader may wonder why the volume of the shell is not taken as: the difference in volume between two concentric spheres. When this expression for the volume is expanded, we obtain (4/3)p(3r²Dr + 3rDr² + Dr³) and for very small values of Dr the terms 3rDr² and Dr³ are negligible in comparison with 3r²Dr. Thus for small values of Dr the two expressions for the volume of the shell approach one another in value and when Dr represents an infinitesimal increment in r they are identical.