Chapter 14, Problem 2RE

The triple integral over a solid G in an $xyz\text{-coordinate}$ system is defined as

${\displaystyle \underset{G}{\iiint }f\left(x,y,z\right)dV=\underset{n\to +\infty }{\lim }{\displaystyle \underset{k=1}{\overset{n}{\sum }}f\left(x_k^{*},y_k^{*},z_k^{*}\right)\Delta V_k}}$

Describe the procedure on which this definition is based.