2025 AIME II Problem 5

Attempt Problem 5 of the 2025 AIME II below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2025 AIME II solutions, or check the answer key.

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5.

Suppose △ABC\triangle ABC has angles ∠BAC=84∘,\angle BAC = 84^\circ, ∠ABC=60∘,\angle ABC = 60^\circ, and ∠ACB=36∘.\angle ACB = 36^\circ. Let D,D, E,E, and FF be the midpoints of sides BC‾,\overline{BC}, AC‾,\overline{AC}, and AB‾,\overline{AB}, respectively. The circumcircle of △DEF\triangle DEF intersects BD‾,\overline{BD}, AE‾,\overline{AE}, and AF‾\overline{AF} at points G,G, H,H, and J,J, respectively. The points G,G, D,D, E,E, H,H, J,J, and FF divide the circumcircle of △DEF\triangle DEF into six minor arcs, as shown. Find DE⌢+2⋅HJ⌢+3⋅FG⌢,\overset{\frown}{DE} + 2 \cdot \overset{\frown}{HJ} + 3 \cdot \overset{\frown}{FG}, where the arcs are measured in degrees.

Answer: 336
Concepts:inscribed angleangle chasingisosceles triangle
Difficulty rating: 2720
Small Hint:

The circle through the midpoints is the nine-point circle, so G,G, H,H, JJ are the feet of the altitudes, and △DEF\triangle DEF has the same angles as △ABC\triangle ABC

Big Hint:

Since ∠BJC=∠BHC=90∘,\angle BJC = \angle BHC = 90^\circ, DD is equidistant from B,B, C,C, H,H, J;J; isosceles triangles turn the arcs into angles of △ABC\triangle ABC

Solution:

The medial triangle DEFDEF has sides parallel to those of ABC,ABC, so ∠FDE=84∘,\angle FDE = 84^\circ, ∠DEF=60∘,\angle DEF = 60^\circ, and ∠DFE=36∘.\angle DFE = 36^\circ. Its circumcircle is the nine-point circle, whose second intersections with the sides of ABCABC are the feet of the altitudes: GG is the foot from A,A, HH the foot from B,B, and JJ the foot from C.C. By the inscribed angle theorem, DE⌢=2∠DFE=72∘.\overset{\frown}{DE} = 2\angle DFE = 72^\circ.

For FG⌢:\overset{\frown}{FG}: since DF‾∥CA‾\overline{DF} \parallel \overline{CA} and GG lies on ray DB,DB, the angle ∠FDG\angle FDG equals the angle between lines CACA and CB,CB, which is 36∘,36^\circ, so FG⌢=2⋅36∘=72∘.\overset{\frown}{FG} = 2 \cdot 36^\circ = 72^\circ. For HJ⌢:\overset{\frown}{HJ}: because ∠BJC=∠BHC=90∘,\angle BJC = \angle BHC = 90^\circ, both HH and JJ lie on the circle with diameter BC‾\overline{BC} centered at D,D, so DJ=DBDJ = DB and DH=DC.DH = DC. Isosceles triangle BDJBDJ gives ∠JDB=180∘−2⋅60∘=60∘,\angle JDB = 180^\circ - 2 \cdot 60^\circ = 60^\circ, and isosceles triangle CDHCDH gives ∠HDC=180∘−2⋅36∘=108∘.\angle HDC = 180^\circ - 2 \cdot 36^\circ = 108^\circ. Hence ∠JDH=180∘−60∘\angle JDH = 180^\circ - 60^\circ −108∘=12∘- 108^\circ = 12^\circ and HJ⌢=24∘.\overset{\frown}{HJ} = 24^\circ.

Therefore DE⌢+2⋅HJ⌢\overset{\frown}{DE} + 2 \cdot \overset{\frown}{HJ} +3⋅FG⌢+ 3 \cdot \overset{\frown}{FG} =72+48+216=336.= 72 + 48 + 216 = 336.

Problem 4#4
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