对角线上的点按顺序标上 A,B,C,D,上面中点为M
AB / BD = 2:3,====AB = 2AD/5
AC :CD = 3:2 === CD = 2AD/5
BC = AD/5 ===== S(GBC) = S(AGD)/5 = S矩形 / 20 = 0.6