TeX source:

\text { (height) } \cdot(\text { base })=\left\{\mathrm{f}\left(\mathrm{c}_{\mathrm{i}}\right)-\mathrm{g}\left(\mathrm{c}_{\mathrm{i}}\right)\right\} \cdot \Delta \mathrm{x}_{\mathrm{i}} \text {. }