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# Long description

The figure consists of a perspective drawing of a three-dimensional set of axes. The two axes in the horizontal plane are labelled x and y. The x-axis points out of the page and to the right; the y-axis points into the page and to the right. The vertical axis is labelled s. The diagram is focused on the section where x, y and s are all positive. The graph of the function u is shown as a convex curved surface shaded blue. The surface is labelled s equals u bracket x comma y close bracket. A point capital P is marked on the surface, and the tangent plane to the surface at the point capital P is shaded in red.