S四邊形OADM = S矩形BDCO-S三角形OBM-S三角形OAD
= 3y -S三角OBM-S三角OAD
s三角形OAD = 3×2/2 = 3
點M的坐標是(6/y，y)。
s三角形OBM = y*(6/y)/2 = 3。
s四邊形OADM = 3y -3 -3 = 6
y = 4
DM = 3/2
BM = 3-3/2 = 3/2
DM = BM