1995 IMO Problems/Problem 1

Problem

Let $A,B,C,D$ be four distinct points on a line, in that order. The circles with diameters $AC$ and $BD$ intersect at $X$ and $Y$ . The line $XY$ meets $BC$ at $Z$ . Let $P$ be a point on the line $XY$ other than $Z$ . The line $CP$ intersects the circle with diameter $AC$ at $C$ and $M$ , and the line $BP$ intersects the circle with diameter $BD$ at $B$ and $N$ . Prove that the lines $AM,DN,XY$ are concurrent.

Hint

Radical axis and radical center! Think RADICAL-ly!

Solution

Since $M$ is on the circle with diameter $AC$ , we have $\angle AMC=90$ and so $\angle MCA=90-A$ . We simlarly find that $\angle BND=90$ . Also, notice that the line $XY$ is the radical axis of the two circles with diameters $AC$ and $BD$ . Thus, since $P$ is on $XY$ , we have $PN\cdotPB=PM\cdot PC$ (Error compiling LaTeX. Unknown error_msg) and so by the converse of Power of a Point, the quadrilateral $MNBC$ is cyclic. Thus, $90-A=\angle MCA=\angle BNM$ . Thus, $\angle MND=180-A$ and so quadrilateral $AMND$ is cyclic. Let the circle which contains the points $AMND$ be cirle $O$ . Then, the radical axis of $O$ and the circle with diameter $AC$ is line $AM$ . Also, the radical axis of $O$ and the circle with diameter $BD$ is line $DN$ . Since the pairwise radical axes of 3 circles are concurrent, we have $AM,DN,XY$ are concurrent as desired.

Solution 2

Let $AM$ and $PT$ (a subsegment of $XY$ ) intersect at $Z$ . Now, assume that $Z, N, P$ are not collinear. In that case, let $ZD$ intersect the circle with diameter $BD$ at $N'$ and the circle through $D, P, T$ at $N''$ .

We know that $<AMC = <BND = <ATP = 90^\circ$ via standard formulae, so quadrilaterals $AMPT$ and $DNPT$ are cyclic. Thus, $N'$ and $N''$ are distinct, as none of them is $N$ . Hence, by Power of a Point, $ZM * ZA = ZP * ZT = ZN'' * ZD.$ However, because $Z$ lies on radical axis $TP$ of the two circles, we have $ZM * ZA = ZN' * ZD.$ Hence, $ZN'' = ZN'$ , a contradiction since $D$ and $D'$ are distinct. We therefore conclude that $Z, N, D$ are collinear, which gives the concurrency of $AM, XY$ , and $DN$ . This completes the problem.

Solution 3

Let $AM$ and $DN$ intersect at $Z$ . Because $<AMC = <BND = <APT = 90^\circ$ , we have quadrilaterals $AMPT$ and $DNPT$ cyclic. Therefore, $Z$ lies on the radical axis of the two circumcircles of these quadrilaterals, so $ZM * ZA = ZN * ZD$ . But as a result $Z$ lies on the radical axis of the two original circles (with diameters $AC$ and $BD$ ), so $Z$ lies on $TP$ as well.

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