2025 AIME II 第 5 题

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5.

ABC\triangle ABC 的三个角为 BAC=84\angle BAC = 84^\circABC=60\angle ABC = 60^\circACB=36\angle ACB = 36^\circ。令 DDEEFF 分别为边 BC\overline{BC}AC\overline{AC}AB\overline{AB} 的中点。DEF\triangle DEF 的外接圆分别与 BD\overline{BD}AE\overline{AE}AF\overline{AF} 交于点 GGHHJJ。点 GGDDEEHHJJFF 如图所示将 DEF\triangle DEF 的外接圆分成六段小弧。求 DE^+2HJ^+3FG^\widehat{DE} + 2 \cdot \widehat{HJ} + 3 \cdot \widehat{FG},其中弧度数以度为单位。

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^+2HJ^+3FG^,\widehat{DE} + 2 \cdot \widehat{HJ} + 3 \cdot \widehat{FG}, where the arcs are measured in degrees.

答案:336
知识点:圆周角导角等腰三角形
难度评级:2720
解答:

中点三角形 DEFDEF 的边分别平行于 ABCABC 的边,所以 FDE=84\angle FDE = 84^\circDEF=60\angle DEF = 60^\circ,且 DFE=36\angle DFE = 36^\circ。它的外接圆是九点圆,与 ABCABC 的边第二次相交于各高的垂足:GG 是从 AA 所作高的垂足,HH 是从 BB 所作高的垂足,JJ 是从 CC 所作高的垂足。由圆周角定理,DE^=2DFE=72\widehat{DE} = 2\angle DFE = 72^\circ

对于 FG^\widehat{FG}:因为 DFCA\overline{DF} \parallel \overline{CA},且 GG 在射线 DBDB 上,所以 FDG\angle FDG 等于直线 CACACBCB 的夹角,也就是 3636^\circ,因此 FG^=236=72\widehat{FG} = 2 \cdot 36^\circ = 72^\circ。对于 HJ^\widehat{HJ}:因为 BJC=BHC=90\angle BJC = \angle BHC = 90^\circ,所以 HHJJ 都在以 BC\overline{BC} 为直径、以 DD 为圆心的圆上,因此 DJ=DBDJ = DBDH=DCDH = DC。等腰三角形 BDJBDJ 给出 JDB=180260=60\angle JDB = 180^\circ - 2 \cdot 60^\circ = 60^\circ,等腰三角形 CDHCDH 给出 HDC=180236=108\angle HDC = 180^\circ - 2 \cdot 36^\circ = 108^\circ。于是 JDH=18060\angle JDH = 180^\circ - 60^\circ 108=12- 108^\circ = 12^\circ,所以 HJ^=24\widehat{HJ} = 24^\circ

因此 DE^+2HJ^\widehat{DE} + 2 \cdot \widehat{HJ} +3FG^+ 3 \cdot \widehat{FG} =72+48+216=336= 72 + 48 + 216 = 336

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^=2DFE=72.\widehat{DE} = 2\angle DFE = 72^\circ.

For FG^:\widehat{FG}: since DFCA\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^=236=72.\widehat{FG} = 2 \cdot 36^\circ = 72^\circ. For HJ^:\widehat{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=180260=60,\angle JDB = 180^\circ - 2 \cdot 60^\circ = 60^\circ, and isosceles triangle CDHCDH gives HDC=180236=108.\angle HDC = 180^\circ - 2 \cdot 36^\circ = 108^\circ. Hence JDH=18060\angle JDH = 180^\circ - 60^\circ 108=12- 108^\circ = 12^\circ and HJ^=24.\widehat{HJ} = 24^\circ.

Therefore DE^+2HJ^\widehat{DE} + 2 \cdot \widehat{HJ} +3FG^+ 3 \cdot \widehat{FG} =72+48+216=336.= 72 + 48 + 216 = 336.

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