Tangents are drawn from any point on the line x+4a=0 to the parabola y2=4ax. Then the angle subtended by  chord of contact at the vertex is

Let Pat12,2at1 and Qat12,2at2 be the points of  contact for tangents drawn from any point on the line x+4a=0 Their point of intersection will be on this line. Therefore, at1t2+4a=0or      t1t2=−4This is also the condition for chord PQ to subtend a right angleat the vertex.