(1)点P的轨迹是一条直线,为抛物线的准线l:y=- ; (2)两切线互相垂直,即PA⊥PB; (3)PF⊥AB; (4)点P的坐标为 . 2.焦点弦:F为抛物线的焦点,AB为抛物线y2=2px(p>0)的焦点弦,A(x1,y1),B(x2,y2),AB的倾斜角为α. (1)x1x2= ; (2)y1y2=-p2; (3)弦长l=x1+x2+p,x1+x2≥2 =p,即当x1=x2时,弦长最短,为2p; (4)弦长l= ;