If the straight line passes through point A (3,0), then b =；;

If the straight line passes through point B (3, 1), then b =；;

If the straight line passes through point C (0, 1), then b =1;

① If the intersection of the straight line and the polyline OAB is on OA, that is, 1≤b≤, as shown in Figure ①.

At this time E(2b, 0)

∴S=OE？ CO=×2b× 1=b

② If the intersection of straight line and polyline OAB is on BA, then < b

At this time, E(3,), d (2b-2, 1).

∴S=S moment -(S△OCD+S△OAE+S△DBE)

=3-[(2b-2)× 1+×(5-2b)？ ()+×3()]=

∴S=.