\( \definecolor{colordef}{RGB}{249,49,84} \definecolor{colorprop}{RGB}{18,102,241} \)
A farmer has 4000 meters of fencing to enclose a rectangular field along a river.Because one side is along the river, fencing is required on only three sides.Let \( x \) be the length perpendicular to the river and \( y \) the length parallel to the river.Find the area \( A \) of the field in terms of \( x \).

\( A = \)
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