International Mathematical Olympiad 1981 Problem 1

$P$ is a point inside a given triangle $ABC$. $D, E, F$ are the feet of the perpendiculars from $P$ to the lines $BC, CA, AB$ respectively. Find all $P$ for which $$\frac{BC}{PD} + \frac{CA}{PE} + \frac{AB}{PF}$$ is least.