Completion requirements

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The figure shows a horizontal line with points x sub k minus 1 and point x sub k marked. The distance between the two points is marked with a double-ended arrow as delta x sub k. and creates the base of a shaded rectangle. Part of the curve y equals f of x is shown and a vertical line from the point x sub k meets the curve and this represents the height of the rectangle. The height of the rectangle is labelled with a double-ended arrow as f of x sub k.