# 1959 IMO Problems/Problem 5

## Problem

An arbitrary point $M$ is selected in the interior of the segment $AB$. The squares $AMCD$ and $MBEF$ are constructed on the same side of $AB$, with the segments $AM$ and $MB$ as their respective bases. The circles about these squares, with respective centers $P$ and $Q$, intersect at $M$ and also at another point $N$. Let $N'$ denote the point of intersection of the straight lines $AF$ and $BC$.

(a) Prove that the points $N$ and $N'$ coincide.

(b) Prove that the straight lines $MN$ pass through a fixed point $S$ independent of the choice of $M$.

(c) Find the locus of the midpoints of the segments $PQ$ as $M$ varies between $A$ and $B$.

## Solution

### Part A

Since the triangles $AFM, CBM$ are congruent, the angles $AFM, CBM$ are congruent; hence $AN'B$ is a right angle. Therefore $N'$ must lie on the circumcircles of both quadrilaterals; hence it is the same point as $N$.

### Part B

We observe that $\frac{AM}{MB} = \frac{CM}{MB} = \frac{AN}{NB}$ since the triangles $ABN, BCM$ are similar. Then $NM$ bisects $ANB$.

We now consider the circle with diameter $AB$. Since $ANB$ is a right angle, $N$ lies on the circle, and since $MN$ bisects $ANB$, the arcs it intercepts are congruent, i.e., it passes through the bisector of arc $AB$ (going counterclockwise), which is a constant point.

### Part C

Denote the midpoint of $PQ$ as $R$. It is clear that $R$'s distance from $AB$ is the average of the distances of $P$ and $Q$ from $AB$, i.e., half the length of $AB$, which is a constant. Therefore the locus in question is a line segment.

## Solution 2

### Part a)

Notice that $\angle BAF = \arctan (\frac{MF}{AM}) = \frac{MB}{AM}$ and $\angle ABC = \arctan (\frac{MC}{MB}) = \frac{AM}{MB}$.

$\implies$ $\angle BAF + \angle ABC = 90^{\circ}$.

Now notice that $\angle FNB = 90^{\circ}$. Considering $\angle BAF$ as $\theta$, this gives $\angle MBN = 90^{\circ} - \theta$ and thus $\angle MFN = 90^{\circ} + \theta$. But notice that $\angle MFA = 90^{\circ} - \theta$, which means that $\angle AFN = 180^{\circ}$. Therefore points $A, F, N$ are collinear. Now $\angle BNF = 90^{\circ}$ and $\angle ANC = 90^{\circ}$. Therefore, $\angle BNC = 180^{\circ}$ and thus points $B, N, C$ are collinear. Therefore, AF and BC intersect at N.

### Part b)

Construct the bisector of arc AB above AB. Call it X. $\angle ANM = \angle ACM = 45^{\circ}$. Now $\angle ANB = 90^{\circ}$ which means N lies on the circle with AB as diameter.

$\implies$ $\angle ANX = \angle ABX = 45^{\circ} = \angle ANM$. Therefore since M and X are on the same side of $N$, $NM$ passes through $X$ wherever we choose $M$ on $AB$.

### Part c)

Let the midpoint of $PQ$ be $R$. Let $G$ be the midpoint of $AM$. Let $F$ be the midpoint of $MB$. Let $I$ be the foot of the perpendicular from $R$ onto $AB$. Therefore by Midpoint Theorem, $RI = \frac{PG + HQ}{2} = \frac{AG + HB}{2} = \frac{AB}{4}$. Therefore the distance $RI$ is a constant and thus the locus is a straight line parallel to $AB$ at a distance (to $AB$, of course) of $\frac{AB}{4}$.