Long description: fig3

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• The image displays a three-dimensional surface defined by a function \(f(x, y)\).
• The horizontal axes represent variables \(x\) and \(y\), with the range for \(x\) extending from \(x = a\) to \(x = b\), and the range for \(y\) extending from an implied lower bound up to a line labeled \(y = d\).
• A portion of the surface is shaded with cross-hatching, indicating the region of integration.
• Several vertical dashed lines indicate the extent of the integration domain: one line corresponds to \(x = a\) and another to \(x = b\). A dashed line labeled \(\delta x\) indicates a small change in the \(x\) direction.
• Another set of vertical dashed lines show the dependence on \(y\), with one line corresponding to \(y = c\) and another extending up to \(y = d\). A vertical dashed line labeled \(\delta y\) is implied by the structure, indicating a small change in the \(y\) direction.
• The axes are labeled with the function \(f(x, y)\) on the vertical axis, and \(x\) and \(y\) on the horizontal axes.