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2025 USAMO Problems/Problem 4

Revision as of 12:34, 22 March 2025 by Mop (talk | contribs)
The following problem is from both the 2025 USAMO #4 and 2025 USAJMO #5, so both problems redirect to this page.

Problem

Let $H$ be the orthocenter of acute triangle $ABC$, let $F$ be the foot of the altitude from $C$ to $AB$, and let $P$ be the reflection of $H$ across $BC$. Suppose that the circumcircle of triangle $AFP$ intersects line $BC$ at two distinct points $X$ and $Y$. Prove that $C$ is the midpoint of $XY$.

Solution 1

Let AP intersects BC at D. Extend FC to the point E on the circumcircle $\omega$ of $AFP$. Since $H$ is the orthocenter of $\Delta ABC$, we know that $HD = DP$ or $HP = 2HD$, and $AH \cdot HD = CH \cdot HF$. Next we use the power of H in $\omega$: $AH \cdot HP = CH \cdot HE$. These relations imply that $HE = 2HF$.

Hence $C, D$ are midpoints of $HE, HP$ respectively. By midline theorem, $CD // EP$. Since $AD \perp CD$, we have $AD \perp EP$. This implies that $\angle APE = 90^{\circ}$. Consequently, $AE$ is the diameter of $\omega$. Let $G$ be the midpoint of $AE$ which is also the center of $\omega$. $G,C$ are midpoints of $AE, EH$ respectively. By the midline theorem again, we have $GC//AH$, consequently, $GC \perp BC$. This implies that $GC$ is the perpendicular bisector of the chord $XY$ hence $C$ is the midpoint of $XY$. ~ Dr. Shi davincimath.com

Solution 2

Denote $O_1$ as the center of $(ABC)$, $O_2$ as the center of $AFP$, $K$ as the midpoint of $AF$, $M$ as the midpoint of $AC$, and $N$ as the midpoint of $AP$. It suffices to show that $\angle{O_2CB}=90$.

Claim: $O_1MO_2C$ is cyclic.

Proof: Since $AK=FK$ and $AM=MC$, KM is a midline of $\triangle{AFC}$ and $KM\parallel FC$. $KO_2\parallel FC$ as well since $\angle{AKO_2}=\angle{AFC}=90$, so $M$ lies on $KO_2$. Next, note that $P$ lies on $(ABC)$, so the perpendicular bisector of $AP$ through $N$ passes through $O_1$. In other words, $N, O_1$, and $O_2$ are collinear. Since $NO_2$ and $BC$ are both perpendicular to $AP$, it follows that they are parallel. Since $KO_2\parallel FC$ and $NO_2\parallel BC$, then $\angle KO_2N=\angle{FCB}$. Finally, we have that \[\angle{MO_2O_1}=\angle{KO_2N}=\angle{FCB}=90-B=\angle{MCO_1},\] and thus $O_1MO_2C$ is cyclic. It follows that $\angle O_1O_2C=\angle{O_1MC}=90$, so $\angle{O_2CB}=180-\angle{O_1MC}=90$, as desired.

-mop

See Also

2025 USAMO (Problems • Resources)
Preceded by
Problem 3 		Followed by
Problem 5
1 • 2 • 3 • 4 • 5 • 6
All USAMO Problems and Solutions
2025 USAJMO (Problems • Resources)
Preceded by
Problem 4 		Followed by
Problem 6
1 • 2 • 3 • 4 • 5 • 6
All USAJMO Problems and Solutions

These problems are copyrighted © by the Mathematical Association of America, as part of the American Mathematics Competitions. AMC Logo.png

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