Olympiad Maths Prep

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Geometry Difficulty 6.5 National olympiad Prove it Turkey

Let ABCABC be a triangle such that the circle ω\omega with diameter BCBC is tangent to the exterior bisector of BAC\angle BAC. The internal bisector of BAC\angle BAC intersects with the side BCBC at point KK and with the circumcircle of ABCABC at point LAL \neq A. Let MM be the midpoint of BCBC. Prove that the circumcircle of KLMKLM is tangent to ω\omega.

Solution

Let Γ\Gamma be the circumcircle of ABCABC. Let the intersection points of the external bisector of BAC\angle BAC with BCBC be TT, with Γ\Gamma be RAR \neq A and with ω\omega be PP. Then, ALAL and PMPM are parallel since both are perpendicular to ATAT, which implies TK/TM=TA/TPTK/TM = TA/TP. Looking at the power of TT with respect to ω\omega and Γ\Gamma we obtain TBTC=TP2=TATRTB \cdot TC = TP^2 = TA \cdot TR therefore TK/TM=TA/TP=TP/TRTK/TM = TA/TP = TP/TR, hence PKRMPK \parallel RM. RMRM is the perpendicular bisector of the side BCBC so PKBCPK \perp BC and PKLMPKLM is a parallelogram with PK=MLPK = ML. Let QQ be the reflection of PP with respect to BCBC, we will show that two circles are tangent to each other at QQ. BCBC is a diameter of ω\omega hence QQ lies on ω\omega. QK=PK=MLQK = PK = ML and both QK,MLQK, ML are perpendicular to BCBC hence KQLMKQLM is a rectangle, so QQ lies on the circumcircle of KLMKLM and it lies on the line passing through the center of both circles, which implies two circles must be tangent to each other at QQ.

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