3.6
BE = 2/3 BC = 4AE = sqrt(AB^2+BE^2) = 5S(ADE) = 1/2 S(ABCD) = 1/2*3*6 = 9(面积)而S(ADE) = 1/2 DM*AE所以DM = 9*2/4 = 4.5