8种情况
(1)A(2,0) B(1,0) C(0,1) y=1/2(x-2)(x-1)
(2)A(2,0) B(1,0) C(0,-1) y=-1/2(x-2)(x-1)
(3)A(2,0) B(-1,0) C(0,1) y=1/2(x-2)(x+1)
(4)A(2,0) B(-1,0) C(0,-1) y=-1/2(x-2)(x+1)
(5)A(-2,0) B(1,0) C(0,1) y=1/2(x+2)(x-1)
(6)A(-2,0) B(1,0) C(0,-1) y=-1/2(x+2)(x-1)
(7)A(-2,0) B(-1,0) C(0,1) y=1/2(x+2)(x+1)
(8)A(-2,0) B(-1,0) C(0,-1) y=-1/2(x+2)(x+1)