$S=y^{2}-4 a x=0, S^{\prime}=y^{2}+a x=0$ are two parabolas and $P(t)$ is a point on the parabola $S^{\prime}=0$. If $A$ and $B$ are the feet of the perpendiculars from $P$ on to coordinate $2 x_{4}$ and $A B$ is a tangent to the parabola $S=0$ at the point $Q\left(t_{1}\right)$, then $t_{1}=$

If the focal chord of the parabola $x^2=12 y$, drawn through the point $(3,0)$ intersects the parabola at the points $P$ and $Q$ then the sum of the reciprocals of the abscissae of the points $P$ and $Q$ is

If the normal drawn at the point $P(9,9)$ on the parabola $y^2=9 x$ meets the parabola again at $Q(a, b)$, then $2 a+b=$

$P$ and $Q$ are the extremities of a focal chord of the parabola $y^2=4 a x$. If $P=(9,9)$ and $Q=(p, q)$, then $p-q=$