OABC各点的坐标：O(0,0),A(3,0),C(0,1),B(3,1)
点D是线段BC上的动点与端点B,C不重合,则y=-1/2x+b与y的截距b的取值范围有限.
y=-1/2x+b过点C时,截距为b=1（b的下限）.
y=-1/2x+b过点B时,1 = -3/2 + b,截距为b=5/2（b的上限）.
1 < b < 5/2
y=-1/2x+b过点A时,0 = -3/2 + b,b = 3/2
1<
OABC各点的坐标：O(0,0),A(3,0),C(0,1),B(3,1)
点D是线段BC上的动点与端点B,C不重合,则y=-1/2x+b与y的截距b的取值范围有限.
y=-1/2x+b过点C时,截距为b=1（b的下限）.
y=-1/2x+b过点B时,1 = -3/2 + b,截距为b=5/2（b的上限）.
1 < b < 5/2
y=-1/2x+b过点A时,0 = -3/2 + b,b = 3/2
1<