1969 Canadian MO Problems/Problem 4

Problem

Let $ABC$ be an equilateral triangle, and $P$ be an arbitrary point within the triangle. Perpendiculars $PD,PE,PF$ are drawn to the three sides of the triangle. Show that, no matter where $P$ is chosen, $\frac{PD+PE+PF}{AB+BC+CA}=\frac{1}{2\sqrt{3}}$ .

Solution

Let a side of the triangle be $s$ and let $[ABC]$ denote the area of $ABC.$ Note that because $2[\triangle ABC]=2[\triangle APB]+2[\triangle APC]+2[\triangle BPC],$ $\frac{\sqrt3}{2}s^2=s(PD+PE+PF).$ Dividing both sides by $s$ , the sum of the perpendiculars from $P$ equals $PD+PE+PF=\frac{\sqrt3}{2}s.$ (It is independant of point $P$ ) Because the sum of the sides is $3s$ , the ratio is always $\cfrac{s\frac{\sqrt3}{2}}{3s}=\frac{1}{2\sqrt3}.$

1969 Canadian MO (Problems)
Preceded by Problem 3	1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 •	Followed by Problem 5

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1969 Canadian MO Problems/Problem 4

Problem

Solution