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Geometry Difficulty 5.3 AIME, harder Prove it Silk Road Mathematics Competition

Convex quadrilateral ABCD is inscribed in circle ω. Rays AB and DC intersect at K. L is chosen on the diagonal BD so that BAC=DAL\angle BAC = \angle DAL. M is chosen on the segment KL so that CMBDCM \parallel BD. Prove that the line BMBM touches ω\omega. (Kungozhin M.)

Figure 1

Solutions — 2

Solution 1

Let N be a point on the line AKAK so that MNALMN \parallel AL. Since CMDL=NMAL=KMKL\frac{CM}{DL} = \frac{NM}{AL} = \frac{KM}{KL} and CMN=DLA\angle CMN = \angle DLA, it follows that KK is a center of homothety which sends CMN\triangle CMN into similar DLA\triangle DLA. On the other hand, DLA\triangle DLA is similar to CBA\triangle CBA because DAL=BAC\angle DAL = \angle BAC and ADL=ACB\angle ADL = \angle ACB. Consequently, (BN,BC)=(MN,MC)\angle (BN, BC) = \angle (MN, MC), and thus points NN, BB, MM, CC are cyclic. Therefore, CBM=CNM=CAB\angle CBM = \angle CNM = \angle CAB from which it follows that BMBM touches ω\omega.

Solution 2

For this solution we need the following theorem.

Pascal's theorem. Points AA, BB, CC, DD, EE, FF (not necessarily in this order) lie on some circle. Then the intersections of the lines ABAB and DEDE, BCBC and EFEF, CDCD and FAFA lie on a straight line.

Back to the problem. Let EE be the second intersection of the line ALAL and ω\omega. Define lbl_b as a tangent line to ω\omega at BB. Let's apply Pascal's theorem on points BB, B1B_1, AA, EE, CC, DD (here B1B_1 coincides with BB) and pairs of lines (BB1BB_1, ECEC), (B1AB_1A, CDCD), (AEAE, DBDB). These pairs of lines coincide with pairs of lines (lbl_b, ECEC), (BABA, CDCD), (AEAE, DBDB). Then from Pascal's theorem, the straight line connecting K=BACDK = BA \cap CD and L=AEDBL = AE \cap DB will also contain lbECl_b \cap EC. Since M=ECKLM = EC \cap KL, it follows that BMBM touches ω\omega, as desired.

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