x-y=1(1)x+2y=3a(2)(2)-(1):3y=3a-1y=a-1/3x=y+1=a+2/3x>0,y<0a+2/3>0a-1/3<0-2/3
先解出该方程组的解。x=2/3+a,y=a-1/3,在根据所给出的不等式关系,得到-2/3