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Geometry Difficulty 6.6 National Olympiad Prove it Taiwan

Let OO be the circumcircle of the triangle ABCABC. Two circles O1O_1, O2O_2 are tangent to each of the circle OO and the rays AB\overrightarrow{AB}, AC\overrightarrow{AC}, with O1O_1 interior to OO, O2O_2 exterior to OO. The common tangent of OO, O1O_1 and the common tangent of OO, O2O_2 intersect at the point XX. Let MM be the midpoint of the arc BCBC (not containing the point AA) on the circle OO, and the segment AA\overline{AA'} be a diameter of OO. Prove that XX, MM, and AA' are collinear.

Solutions — 2

Solution 1

(Solution 1): Let the point of tangency of OO, O1O_1 be PP, and the point of tangency of OO, O2O_2 be QQ; the original problem is equivalent to PMQAPMQA' being a harmonic quadrilateral. Consider the transformation "inversion centered at AA with power AB×AC\overrightarrow{AB} \times \overrightarrow{AC}, followed by reflection across the angle bisector of BAC\angle BAC"; then AA, BB, CC remain fixed, O1O_1 is transformed into the AA-excircle (let its center be IAI_A), and O2O_2 is transformed into the incircle (let its center be II). Let DD, YY, ZZ be the feet of the perpendiculars from AA, II, IAI_A respectively to BC\overrightarrow{BC}, and let LL be the intersection of the angle bisector of BAC\angle BAC with BC\overrightarrow{BC}; then PP is transformed to ZZ, QQ is transformed to YY, AA' is transformed to DD, and MM is transformed to LL; the original problem is equivalent to (D,L;Y,Z)(D, L; Y, Z) being a harmonic range, which is again equivalent to (A,L;I,IA)(A, L; I, I_A) being a harmonic range. Since AI:IL=AB:BL=AIA:LIA\overrightarrow{AI} : \overrightarrow{IL} = \overrightarrow{AB} : \overrightarrow{BL} = \overrightarrow{AI_A} : \overrightarrow{LI_A}, the original proposition holds.

Figure 1

Let MAMA' meet ABAB at M1M_1, meet ACAC at M2M_2, let O1O_1 be tangent to ABAB at N1N_1, O1O_1 be tangent to ACAC at N2N_2, O2O_2 be tangent to ABAB at W1W_1, O2O_2 be tangent to ACAC at W2W_2. By Mannheim's theorem, N1N_1, II, N2N_2 are collinear, W1IAW2W_1I_AW_2 are collinear, and N1N2AIN_1N_2 \perp AI, W1W2AIAW_1W_2 \perp AI_A. By the Incenter-Excenter Lemma, IM=IAMIM = I_AM, hence N1M1=W1M1=N2M2=W2M2N_1M_1 = W_1M_1 = N_2M_2 = W_2M_2, from which it follows that MAMA' is indeed the radical axis of O1O_1, O2O_2. This completes the proof.

Figure 1

Solution 2

It suffices to prove that MAMA' is the radical axis of O1O_1, O2O_2; then XX is the radical center of the three circles, and the original proposition is thereby proved.
Let MAMA' meet ABAB at M1M_1, meet ACAC at M2M_2, let O1O_1 be tangent to ABAB at N1N_1, O1O_1 be tangent to ACAC at N2N_2, O2O_2 be tangent to ABAB at W1W_1, O2O_2 be tangent to ACAC at W2W_2. By Mannheim's theorem, N1N_1, II, N2N_2 are collinear, W1IAW2W_1I_AW_2 are collinear, and N1N2AIN_1N_2 \perp AI, W1W2AIAW_1W_2 \perp AI_A. By the Incenter-Excenter Lemma, IM=IAMIM = I_AM, hence N1M1=W1M1=N2M2=W2M2N_1M_1 = W_1M_1 = N_2M_2 = W_2M_2, from which it follows that MAMA' is indeed the radical axis of O1O_1, O2O_2. This completes the proof.

Figure 1

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