当P在CD上时x+y=1,当P在BC上时AP=(1-y)AB+yAC=2(1-y)AD+yAC,∴x/2+y=1.看图得x+y>=1,x/2+y<=1.由x,y>=0得0<=x<=2,0<=y<=1.
泠泠七弦静听松风寒
