设 A(x1,y1) 、B(x2,y2) ,则有：y1² = 4x1 ,y2² = 4x2 ,依题意,可设直线AB的方程为 x+my = b（b≠0）,则有：x = b-my ,代入 y² = 4x ,可得：y²+4my-4b = 0 ；由韦达定理可得：y1y2 = -4b ,则有...