If P, Q, R are three points on a parabola y2 = 4ax whose ordinates are in geometrical progression, then the tangents at P and R meet on

Let the coordinates of P, Q, R be (at2 , 2at ) i =1,2,3 having ordinates in G.P. So that t1, t2, t3 are also in G.P. i.e. t1t3 = t22 . Equations of the tangents at P and R are t1y=x+at12  and t3y=x+at32, which intersect at the point x+at12t1=x+at32t3 ⇒x=at1t3=at22  which is a line through Qparallel to y-axis.