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way of graphically representing the solutions of a first-order differential equation without actually solving the equation. The equation y' = f (x,y) gives a direction, y', associated with each point (x,y) in the plane that must be satisfied by any solution curve passing through that point. The direction field is defined as the collection of small line segments passing through various points having a slope that will satisfy the given differential equation (see ) at that point. The actual family of curves (solutions of the differential equation) must have a direction at each point that agrees with that of the line segment of the direction field at that point, so that this method is valuable for gaining some feeling for the behaviour of the solutions in cases in which the equation is difficult to solve or in which the solution is a complicated function. Often it is helpful when drawing the direction field to determine the lines or curves, called isoclines, on which the slope of the direction field segments is constant. For example, in the equation y' = x + y the slope will have the constant value k when k = x + y, or when y = -x + k; that is, the isoclines are straight lines with a slope of -1. These lines can then be sketched in lightly to aid in constructing the direction field (see Graph). The actual family of solutions in this case is y = aex - x - 1 for any constant a, as found by methods of differential equations.