$17 \cdot 90$ As shown, $\triangle A B C$ is an equilateral triangle, $D E=19, D F=89$, then the perimeter of $\triangle A B C$ is
(A) 216.
(B) $216 \sqrt{3}$.
(C) 648.
(D) $324 \sqrt{3}$.
(1st "Five Sheep Cup" Junior High School Mathematics Competition, 1989)
［Solution］Suppose $\triangle A B C$ is an equilateral triangle, then $D E+D F=h$ (the height on one side), which can be derived using the area formula.
Therefore,
$$
\begin{array}{l} 
\text { Hence } h=19+89=108 . \\
\text { Then } \quad 3 a=3 \cdot \frac{2}{\sqrt{3}} h=2 h \sqrt{3} \\
= 216 \sqrt{3} .
\end{array}
$$

Therefore, the answer is $(B)$.