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Geometry Difficulty 6.5 National olympiad Prove it Iran

Let ABC\triangle ABC be an isosceles triangle (AB=ACAB = AC) with incenter II. Circle ω\omega passes through CC and II and is tangent to AIAI. The circle ω\omega intersects ACAC and circumcircle of ABC\triangle ABC at QQ and DD, respectively. Let MM be the midpoint of ABAB and NN be the midpoint of CQCQ. Prove that ADAD, MNMN and BCBC are concurrent.

Solution

Let PP be the midpoint of segment BCBC and JJ be the midpoint of arc \widearcBC\widearc{BC} (JAJ \neq A).
Figure 1
We call the circumcircle of triangle CID\triangle CID, ω\omega and the intersection point of ω\omega and segment BCBC, RR. We have
QIC=180(IQC+ICQ)=180(PIC+ICP)=90. \angle QIC = 180^{\circ} - (\angle IQC + \angle ICQ) = 180^{\circ} - (\angle PIC + \angle ICP) = 90^{\circ}.
So, QIC=90\angle QIC = 90^{\circ} and NN is the center of ω\omega which gives us QRC=90\angle QRC = 90^{\circ} and QRAPQR \parallel AP. RDC=RQC\angle RDC = \angle RQC gives us JAC=JDC\angle JAC = \angle JDC. So, points D,RD, R and JJ are collinear. Since RNC=2RQC=2PAC=A\angle RNC = 2\angle RQC = 2\angle PAC = \angle A, we have RNABRN \parallel AB. Therefore
ANNC=BRRC    ANNCRCBRBMMA=1. \frac{AN}{NC} = \frac{BR}{RC} \implies \frac{AN}{NC} \cdot \frac{RC}{BR} \cdot \frac{BM}{MA} = 1.
So, the lines ARAR, BNBN and CMCM are concurrent. Let XX be the intersection point of lines ADAD and BCBC. It suffices to show that (XR,CB)=1(XR, CB) = -1. Since D,RD, R and JJ are collinear and JJ is the midpoint of arc \widearcBC\widearc{BC}, We have
(XR,CB)=D(AJ,CB)=1. (XR, CB) \stackrel{D}{=} (AJ, CB) = -1.
Hence the result.

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