IMO 1981 Problem 1

Consider a variable point $P$ inside a given triangle $ABC$ . Let $D$ , $E$ , $F$ be the feet of the perpendiculars from the point $P$ to the lines $BC$ , $CA$ , $AB$ , respectively. Find all points $P$ which minimize the sum $${BC\over PD}+{CA\over PE}+{AB\over PF}.$$