17. As shown in Figure 3, in trapezoid $A B C D$, $F$ is the midpoint of $C D$, $A F \perp A B$, $E$ is a point on side $B C$ such that $A E=B E$. If $A B=m$ (where $m$ is a constant), then the length of $E F$ is $\qquad$
17. $\frac{m}{2}$.

As shown in Figure 3, extend $A F$ to intersect the extension of $B C$ at point $G$. It is easy to prove that $\triangle A D F \cong \triangle G C F$. Therefore, $A F = G F$.

Since $\angle B A G = 90^{\circ}$ and $A E = B E$, $A E$ is the median of the hypotenuse $B G$ in the right triangle $\triangle A B G$. Thus, $E F$ is the midline of $\triangle A B G$. Hence, $E F = \frac{1}{2} A B = \frac{m}{2}$.