2006 Canadian MO Problems/Problem 2


Let $ABC$ be an acute angled triangle. Inscribe a rectangle $DEFG$ in this triangle so that $D$ is on $AB$, $E$ on $AC$, and $F$ and $G$ on $BC$. Describe the locus of the intersections of the diagonals of all possible rectangles $DEFG$.


The locus is the line segment which joins the midpoint of side $BC$ to the midpoint of the altitude to side $BC$ of the triangle.

Let $r = \frac{AD}{AB}$ and let $H$ be the foot of the altitude from $A$ to $BC$. Then by similarity, $\frac{AE}{AC} = \frac{GH}{BH} = \frac{FH}{CH} = r$.

Now, we use vector geometry: intersection $I$ of the diagonals of $DEFG$ is also the midpoint of diagonal $DF$, so

$I = \frac{1}{2}(D + F) = \frac{1}{2}((rA + (1 - r)B) + (rH + (1 - r)C)) = r \frac{A + H}{2} + (1 - r)\frac{B + C}{2}$,

and this point lies on the segment joining the midpoint $\frac{A + H}{2}$ of segment $AH$ and the midpoint $\frac{B + C}{2}$ of segment $BC$, dividing this segment into the ratio $r : 1 - r$.

Solution 2

We claim that the desired locus is the line segment from the midpoint $A'$ of altitude $AD$ to the midpoint of $BC$, $M$, not including both endpoints.

A homothety about $A$ maps the rectangle $DEFG$ onto rectangle $BCHI$ in the exterior of $ABC$. The scale factor of the homothety is $\frac{AD}{AB}$, which is also the scale factor of the mapping of the intersection of diagonals (the original we call $X$ and the new we call $Y$. Hence $\frac{AX}{XY} = \frac{AD}{DB}$. But $\frac{AA'}{MY} = \frac{AH}{BI} = \frac{\frac{AH}{DG}}{\frac{BI}{DG}} = \frac{AD}{BD}$, and $MY // AH$, so $MYX$ and $A'AX$ are similar, and so $X$ lies on $A'M$, as desired. Reversing the argument proves the other direction for a locus, and we are done.

See also

2006 Canadian MO (Problems)
Preceded by
Problem 1
1 2 3 4 5 Followed by
Problem 3