2014 USAJMO Problems/Problem 2

Problem

Let $\triangle{ABC}$ be a non-equilateral, acute triangle with $\angle A=60^{\circ}$ , and let $O$ and $H$ denote the circumcenter and orthocenter of $\triangle{ABC}$ , respectively.

(a) Prove that line $OH$ intersects both segments $AB$ and $AC$ .

(b) Line $OH$ intersects segments $AB$ and $AC$ at $P$ and $Q$ , respectively. Denote by $s$ and $t$ the respective areas of triangle $APQ$ and quadrilateral $BPQC$ . Determine the range of possible values for $s/t$ .

Solution

$[asy] import olympiad; unitsize(1inch); pair A,B,C,O,H,P,Q,i1,i2,i3,i4; //define dots A=3*dir(50); B=(0,0); C=right*2.76481496; O=circumcenter(A,B,C); H=orthocenter(A,B,C); i1=2*O-H; i2=2*i1-O; i3=2*H-O; i4=2*i3-H; //These points are for extending PQ. DO NOT DELETE! P=intersectionpoint(i2--i4,A--B); Q=intersectionpoint(i2--i4,A--C); //draw dot(P); dot(Q); draw(P--Q); dot(A); dot(B); dot(C); dot(O); dot(H); draw(A--B--C--cycle); //label label("$A$",A,N); label("$B$",B,SW); label("$C$",C,SE); label("$P$",P,NW); label("$Q$",Q,NE); label("$O$",O,N); label("$H$",H,N); //change O and H label positions if interfering with other lines to be added //further editing: ABCPQOH are the dots to be further used. i1,i2,i3,i4 are for drawing assistence and are not to be used [/asy]$

Claim: $H$ is the reflection of $O$ over the angle bisector of $\angle BAC$ (henceforth 'the' reflection)

Proof: Let $H'$ be the reflection of $O$ , and let $B'$ be the reflection of $B$ .

Then reflection takes $\angle ABH'$ to $\angle AB'O$ .

$\Delta ABB'$ is equilateral, and $O$ lies on the perpendicular bisector of $\overline{AB}$

It's well known that $O$ lies strictly inside $ABC$ , meaning that $\angle ABH' = \angle AB'O = 30^{\circ},$ from which it follows that $\overline{BH'} \perp \verline{AC}$ (Error compiling LaTeX. Unknown error_msg) . Similarly, $\overline{CH'} \perp \verline{AB}$ (Error compiling LaTeX. Unknown error_msg). Since $H'$ lies on two altitudes, $H$ is the orthocenter, as desired.

So $\overline{OH}$ is perpendicular to the angle bisector of $\angle OAH$ , which is the same line as the angle bisector of $\angle BAC$ , meaning that $\Delta APQ$ is equilateral.

Let its side length be $s$ , and let $PH=t$ , where $0 < t < s, t \neq s/2$ because O lies strictly within $\angle BAC$ , as must $H$ , the reflection of $O$ . Also, it's easy to show that if $O=H$ in a general triangle, it's equlateral, and we know that $\Delta ABC$ is not equilateral. Hence $t \neq s/2$ . Let $\overrightarrow{BH}$ intersect $\overline{AC}$ at $P_B$ .

Since $\Delta HP_BQ$ and $BP_BA$ are 30-60-90 triangles, $AB=2AP_B=2(s-QP_B)=2(s-HQ/2)=2s-HQ=2s-(s-t)=s+t$

Similarly, $AC=2s-t$

The ratio $\frac{[APQ]}{[ABC]}$ is $\frac{AP AQ}{AB AC} = \frac{s^2}{(s+t)(2s-t)}$ The denominator equals $(1.5s)^2-(.5s-t)^2$ where $.5s-t$ can equal any value in $(-.5s, .5s)$ except $0$ . Therefore, the denominator can equal any value in $(2s^2, 9s^2/4)$ , and the ratio is any value in $\boxed{\left(\frac{4}{9},\frac{1}{2}\right)}.$

Note: It's easy to show that for any point $H$ on $\verline{PQ}$ (Error compiling LaTeX. Unknown error_msg), Points B and C can be validly defined to make an acute, non-equilateral triangle

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2014 USAJMO Problems/Problem 2

Problem

Solution