In a circle, the central angle $d\mathrm{\θ}$ intercepts an arc of length $r d\mathrm{\θ}$. Taking the "wedge" shown in Figure 7.2.1 as a triangle, its area is half the base times the height. The base is the arc, approximated by $r d\mathrm{\θ}$ and the altitude is approximated by $r$, the length of either side of the angle $d\mathrm{\θ}$. Hence, the area of the "wedge" in Figure 7.2.1 is taken as $\frac{1}{2}\left(r d\mathrm{\θ}\right)r\=\frac{1}{2}{r}^{2}d\mathrm{\θ}$.