If a region's bounding curves are described by functions of [math]y[/math], the approximating rectangles are horizontal instead of vertical and the basic formula has [math]y[/math] in place of [math]x[/math]. This interactive figure shows the integral [math]\int_c^d\left[f\left(y\right)-g\left(y\right)\right]dy[/math] and how it arises from a limit of approximating rectangles. When [math]f[/math] is to the right of [math]g[/math] (and when [math]c \lt d[/math]) the integral will be positive.

[i][color=#444444]Developed for use with Pearson calculus publications.[/color][/i]