x(b+1/2a)=OC
OC=BC+yBD
BD=a-b,BC=b-1/2a
联立方程得:(1-y)b+(y-1/2)a=xb+1/2xa(利用OC)
然后用待定系数:(1-y)=x,(y-1/2)=1/2x
解得x=1/6,y=2/3