For the sake of illustration, let us assume we cannot or do
not want to solve this analytically (even though it is
separable). Therefore, we do not know a function y(x) that
satisfies (4). Again, if we did, such a function would be the
analytic solution to (4). We do have information about the
function, specifically the derivative of y for all points
(x,y). For example, the derivative of y at the point (2,3) is
-4/9. Recall that a derivative is the slope of the tangent
line to the curve at that point. Therefore, let us draw an
arrow with its tail at the point (2,3) and a slope of -4/9.
What can this do for us? Let us repeat this process again for
the points (1,2), (1,3), and (2,2) which produces the picture
to the left below. We have produced a graph that is called a vector
field or direction field. Whether this is
helpful at this point may be a debatable topic. Let us repeat
this process for more points which produces the vector field
to the right below. Do you have a better sense of the
direction of the underlying solutions?
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