18.50 As shown in the figure, in rectangle $A B C D$, it is known that $A B=8, \quad B C=22$, $E, F$ are points on $B C$, and $A E=E F=F D$. Then the length of $E F$ is
(A) 9.
(B) 10.
(C) 11.
(D) 12.
(China Sichuan Province Junior High School Mathematics Competition, 1991)
[Solution] Let $A E=x$, then $B E=11-\frac{x}{2}$. From

we have
$$
A E^{2}-A B^{2}=B E^{2},
$$

which is
$$
\begin{array}{c}
x^{2}-64=\left(11-\frac{x}{2}\right)^{2}, \\
3 x^{2}+44 x-740=0 .
\end{array}
$$

Solving this, we get $x=10$ or $-\frac{74}{3}$ (discard). Thus, $E F=10$. Therefore, the answer is $(B)$.