TeX source:

\begin{aligned} &\text { (height of the slice) } \cdot \text { (base of the slice)\cdot(thickness) } \\ &\approx\left(\mathrm{c}_{\mathrm{i}}+1\right) \cdot\left(\mathrm{c}_{\mathrm{i}}+1\right) \cdot \Delta \mathrm{x}_{\mathrm{i}} \end{aligned}