xx1 = 2a(y+y1) ... (1)
xx2 = 2a(y+y2) ... (2)
Let T(x0,y0) be external point and A(x1,y1), B(x2,y2) two points on parabola).
(1) and (2) are equations of tangents.
(1)...
T must lie on tangent (1)
x0x1=2a(y0+y1)
THUS A must lie on the line.
x0x = 2a(y0 + y) ...(3)
(2)...
T...