p=-3x+y+2z=-3x+y+2(1-x-y)=2-5x-y
q=x-2y+4z=q=x-2y+4(1-x-y)=4-3x-6y
点(p,q)=(2-5x-y,4-3x-6y) 在x>=0,y>=0,x+y=0,y>=0,x+y