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Geometry Difficulty 5.6 AIME, harder Prove it Hong Kong

In acute ABC\triangle ABC, BC>ABBC > AB. Let DD be the point on the side ACAC such that BD=BABD = BA. Let MM be the midpoint of BCBC. The tangents at DD and MM to the circumcircle of CDM\triangle CDM intersect at point EE. The line BEBE intersects the side ACAC at FF. Prove that A,B,M,FA, B, M, F are concyclic.

Solutions — 2

Solution 1

Suppose BDBD meets (CDMCDM) again at PP. Applying Pascal's theorem to the points MMCDDPMMCDDP, we know that MMDD=EMM \cap DD = E, MCDP=BMC \cap DP = B and CDPMCD \cap PM are collinear. Thus, MPMP passes through BECD=FBE \cap CD = F. Now, since
CMF=ADB=BAF, \angle CMF = \angle ADB = \angle BAF,
the points A,B,M,FA, B, M, F are concyclic.

Figure 1

Solution 2

We provide another proof without using Pascal's theorem as follows. Again, the main task is to prove M,P,FM, P, F are collinear. We define FF' as the intersection point of CDCD and MPMP. It suffices to prove B,E,FB, E, F' are collinear. Suppose DEDE meets (PDFPDF') again at XX, and MEME meets (MCFMCF') again at YY.
Firstly, since BP×BD=BM×BCBP \times BD = BM \times BC, the powers of BB with respect to (PDFPDF') and (MCFMCF') are the same. Thus, BB lies on the radical axis of these circles.
Secondly, as XFP=XDP=DMP=DMF\angle XF'P = \angle XDP = \angle DMP = \angle DMF', we have XF//MDXF'//MD. Also, as MYF=180MCF=180YMD\angle MYF' = 180^\circ - \angle MCF' = 180^\circ - \angle YMD, we have YF//MDYF'//MD. Therefore, Y,X,FY, X, F' are collinear, and YX//MDYX//MD. Since EM=EDEM = ED, we have EY=EXEY = EX. From this, it follows that EX×ED=EY×EMEX \times ED = EY \times EM, and hence EE lies on the radical axis of (PDFPDF') and (MCFMCF').
Thirdly, it is clear that FF' lies on the radical axis of (PDFPDF') and (MCFMCF'). It follows that B,E,FB, E, F' are collinear as desired.

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