7. A small volume element dV is shown in the figure. ZA As de -+ 0, dr -> 0 and dz - 0 this small piece dV approaches a rectangular box. (a) Derive expressions for , B and 6 in terms of r, dr, do and dz do B (b) Use (a) to derive a approximate formula for dV in terms of dr, do and dz as these quantities approach zero