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Geometry Difficulty 6.6 National Olympiad Prove it North Macedonia

Let ABCABC be an acute triangle such that ABACAB \neq AC, with circumscribed circle Γ\Gamma and circumcenter OO. Let MM be the midpoint of BCBC and DD be a point on Γ\Gamma such that ADBCAD \perp BC. Let TT be a point such that BDCTBDCT is a parallelogram and QQ a point on the same side of BCBC as AA, such that
BQM=BCA and CQM=CBA. \angle BQM = \angle BCA \text{ and } \angle CQM = \angle CBA.
Let the line AOAO intersect Γ\Gamma at EE, (EAE \neq A) and let the circumscribed circle of ETQ\triangle ETQ intersect Γ\Gamma at point XEX \neq E. Prove that the points A,MA, M and XX are collinear.

Solutions — 2

Solution 1

Let XX' be the symmetric point to QQ in line BCBC. Now since CBA=CQM=CXM\angle CBA = \angle CQM = \angle CX'M, BCA=BQM=BXM\angle BCA = \angle BQM = \angle BX'M, we have
BXC=BXM+CXM=CBA+BCA=180BAC \angle BX'C = \angle BX'M + \angle CX'M = \angle CBA + \angle BCA = 180^\circ - \angle BAC
we have that XΓX' \in \Gamma. Now since AXB=ACB=MXB\angle AX'B = \angle ACB = \angle MX'B we have that A,M,XA, M, X' are collinear. Note that since
DCB=DAB=90ABC=OAC=EAC \angle DCB = \angle DAB = 90^\circ - \angle ABC = \angle OAC = \angle EAC
we get that DBCEDBCE is an isosceles trapezoid.
Since BDCTBDCT is a parallelogram we have MT=MDMT=MD, with M,D,TM, D, T being collinear, BD=CTBD=CT, and since BDECBDEC is an isosceles trapezoid we have BD=CEBD=CE and ME=MDME=MD. Since
BTC=BDC=BED,CE=BD=CT and \angle BTC = \angle BDC = \angle BED, \quad CE = BD = CT \text{ and}
Figure 1
ME=MTME=MT we have that EE and TT are symmetric with respect to the line BCBC. Now since QQ and XX' are symmetric with respect to the line BCBC as well, this means that QXETQX'ET is an isosceles trapezoid which means that Q,X,E,TQ, X', E, T are concyclic. Since XΓX' \in \Gamma this means that XXX \equiv X' and therefore A,M,XA, M, X are collinear.

Solution 2

Denote by HH the orthocenter of ABC\triangle ABC. We use the following well known properties:
(i) Point DD is the symmetric point of HH with respect to BCBC. Indeed, if H1H_1 is the symmetric point of HH with respect to BCBC then BH1C+BAC=180\angle BH_1C + \angle BAC = 180^\circ and therefore H1DH_1 \equiv D.
(ii) The symmetric point of HH with respect to MM is the point EE. Indeed, if H2H_2 is the symmetric point of HH with respect to MM then BH2CHBH_2CH is a parallelogram, BH2C+BAC=180\angle BH_2C + \angle BAC = 180^\circ and since EBCHEB \parallel CH we have EBA=90\angle EBA = 90^\circ.
Since DETHDETH is a parallelogram and MH=MDMH=MD we have that DETHDETH is a rectangle. Therefore MT=MEMT=ME and TEBCTE \perp BC implying that TT and EE are symmetric with respect to BCBC. Denote by QQ' the symmetric point of QQ with respect to BCBC. Then QETQQ'ETQ is an isosceles trapezoid, so QQ' is a point on the circumscribed circle of ETQ\triangle ETQ. Moreover BQC+BAC=180\angle BQ'C + \angle BAC = 180^\circ and we conclude that QΓQ' \in \Gamma. Therefore QXQ' \equiv X.
It remains to observe that CXM=CQM=CBA\angle CXM = \angle CQM = \angle CBA and CXA=CBA\angle CXA = \angle CBA and we infer that X,MX, M and AA are collinear.

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