5. A line $l$ passes through the focus $F$ of the parabola $y^{2}=4 p x(p>0)$ and intersects the parabola at points $P, Q$. Perpendicular lines $P R, Q S$ are drawn from points $P, Q$ to the directrix, with feet of the perpendiculars being $R, S$. Let $|P F|=a,|Q F|=b, M$ be the midpoint of $R S$. Find the length of $|M F|$.
5. By the definition of a parabola, we know $|P R|=|P F|,|Q S|=|Q F|$, thus $\angle P R F=\angle P F R, \angle Q F S=\angle F S Q$, which leads to $\angle R F S=90^{\circ}$. Also, $R S^{2}=(a+b)^{2}-(a-b)^{2}=4 a b$, so $|R S|=2 \sqrt{a b}$. Since $M$ is the midpoint of $R S$, we have $|M F|=\frac{1}{2}$ $|R S|=\sqrt{a b}$.