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Geometry Difficulty 6.5 National olympiad Prove it Turkey

Let ABCABC be a triangle and let PP be a point in the interior of this triangle. Let ωA\omega_A be the circle that is tangent to the circumcircle of BPCBPC at PP internally and tangent to the circumcircle of ABCABC at A1A_1 internally. Let ΓA\Gamma_A be the circle that is tangent to the circumcircle of BPCBPC at PP externally and tangent to the circumcircle of ABCABC at A2A_2 internally. Define B1,B2,C1,C2B_1, B_2, C_1, C_2 similarly. Let OO be the circumcentre of ABCABC. Prove that the lines A1A2,B1B2,C1C2A_1A_2, B_1B_2, C_1C_2 and OPOP are concurrent.

Solution

Figure 1
From the radical axis theorem on the circles (ABC)(ABC), (BCP)(BCP) and ωA\omega_A, the tangent lines to ωA\omega_A passing through PP and A1A_1 intersect on the line BCBC, let us say at point DD. From the radical axis theorem on the circles (ABC)(ABC), (BCP)(BCP) and ΓA\Gamma_A, we see that the line passing through A2A_2 and tangent to the circle (ABC)(ABC) passes through DD as well. Therefore, DD is the pole of the line A1A2A_1A_2. Define the points E,FE, F analogously. Looking at the similarity DBPDPCDBP \sim DPC we have

BPPC=DBDP=DPDCand hence(BPPC)2=DBDC \frac{BP}{PC} = \frac{DB}{DP} = \frac{DP}{DC} \quad \text{and hence} \quad \left(\frac{BP}{PC}\right)^2 = \frac{DB}{DC}

(CPPA)2=ECEAand(APPB)2=FAFB \left(\frac{CP}{PA}\right)^2 = \frac{EC}{EA} \quad \text{and} \quad \left(\frac{AP}{PB}\right)^2 = \frac{FA}{FB}
Therefore, by Menelaus' Theorem we get that the points D,E,FD, E, F are collinear. Thus, from La Hire Theorem we can deduce that the lines A1A2A_1A_2, B1B2B_1B_2 and C1C2C_1C_2 are concurrent. Let the common point of these lines be MM. Since MM is the pole of the line DEF\overline{DEF} we get OMDEOM \perp DE. Therefore, in order to complete the solution we should prove that O,M,PO, M, P are collinear. Hence it suffices to show that OPOP is perpendicular to DEDE. Let rr be the circumradius of the triangle ABCABC. From the power of the point DD with respect to the circle (ABC)(ABC) we have DP2=DBDC=DO2r2DP^2 = DB \cdot DC = DO^2 - r^2. Similarly, from the power of the point EE with respect to the circle (ABC)(ABC) we get EP2=EO2r2EP^2 = EO^2 - r^2. Hence DP2+EO2=DO2+EP2DP^2 + EO^2 = DO^2 + EP^2 and we get OPDEOP \perp DE. We are done.

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