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Geometry Difficulty 5.4 AIME, harder Prove it Taiwan

在三角形 ABCABC 的內部選一個點 TT。令 A1,B1,C1A_1, B_1, C_1 分別為 TT 對直線 BC,CA,ABBC, CA, AB 的反射點。記三角形 A1B1C1A_1B_1C_1 的外接圓為 Ω\Omega。設直線 A1T,B1T,C1TA_1T, B_1T, C_1T 分別與圓 Ω\Omega 再交於點 A2,B2,C2A_2, B_2, C_2。證明: 直線 AA2,BB2,CC2AA_2, BB_2, CC_2 共點, 且其交點在 Ω\Omega 上。

Solution

By (l,n)\angle (\boldsymbol{l}, n) we always mean the directed angle of the lines ll and nn, taken modulo 180180^\circ.

Figure 1

Let CC2CC_2 meet Ω\Omega again at KK (as usual, if CC2CC_2 is tangent to Ω\Omega, we set K=C2K = C_2). We show that the line BB2BB_2 contains KK; similarly, AA2AA_2 will also pass through KK. For this purpose, it suffices to prove that
(C2C,C2A1)=(B2B,B2A1).(1) \angle(C_2C, C_2A_1) = \angle(B_2B, B_2A_1). \qquad (1)
By the problem condition, CBCB and CACA are the perpendicular bisectors of TA1TA_1 and TB1TB_1, respectively. Hence, CC is the circumcenter of the triangle A1TB1A_1TB_1. Therefore,
(CA1,CB1)=(CB,CT)=(B1A1,B1T)=(B1A1,B1B2). \angle(CA_1, CB_1) = \angle(CB, CT) = \angle(B_1A_1, B_1T) = \angle(B_1A_1, B_1B_2).

In circle Ω\Omega we have (B1A1,B1B2)=(C2A1,C2B2)\angle(B_1A_1, B_1B_2) = \angle(C_2A_1, C_2B_2). Thus,
(CA1,CB)=(B1A1,B1B2)=(C2A1,C2B2).(2) \angle(CA_1, CB) = \angle(B_1A_1, B_1B_2) = \angle(C_2A_1, C_2B_2). \qquad (2)
Similarly, we get
(BA1,BC)=(C1A1,C1C2)=(B2A1,B2C2).(3) \angle(BA_1, BC) = \angle(C_1A_1, C_1C_2) = \angle(B_2A_1, B_2C_2). \qquad (3)
The two obtained relations yield that the triangles A1BCA_1BC and A1B2C2A_1B_2C_2 are similar and equioriented, hence
A1B2A1B=A1C2A1Cand(A1B,A1C)=(A1B2,A1C2). \frac{A_1B_2}{A_1B} = \frac{A_1C_2}{A_1C} \quad \text{and} \quad \angle(A_1B, A_1C) = \angle(A_1B_2, A_1C_2).
The second equality may be rewritten as (A1B,A1B2)=(A1C,A1C2)\angle(A_1B, A_1B_2) = \angle(A_1C, A_1C_2), so the triangles A1BB2A_1BB_2 and A1CC2A_1CC_2 are also similar and equioriented. This establishes (1). \Box

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Source: MathNet, licensed CC-BY-4.0. Statement translated into English from zh; metadata (topic, difficulty) added by this project.