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Geometry Difficulty 6.7 National Olympiad Prove it Mongolia

Let AA be one of the intersection points of circles ω1(O1)\omega_1(O_1) and ω2(O2)\omega_2(O_2). \ell is a line that touches ω1\omega_1 and ω2\omega_2 at BB and CC respectively. Let O3O_3 be the center of the circumscribed circle of ABCABC. And we choose DD as AA is midpoint of O3DO_3D. If MM is midpoint of O1O2O_1O_2, then prove that O1DM=O2DA\angle O_1DM = \angle O_2DA.

Solution

Let us denote by QQ, RR and SS the midpoints of BCBC, ABAB and ACAC respectively. Then O3RABO_3R \perp AB, O1RABO_1R \perp AB, O3SACO_3S \perp AC, O2SACO_2S \perp AC, O3QBCO_3Q \perp BC and MQBCMQ \perp BC. Assuming ABC=β\angle ABC = \beta and BCA=γ\angle BCA = \gamma, we get BO3O1=γ\angle BO_3O_1 = \gamma and CO3O2=β\angle CO_3O_2 = \beta, as O3O_3 is the circumcenter of ABC\triangle ABC.

Figure 1

Since BCBC is tangent to ω1\omega_1, A~B=β\tilde{A}B = \beta. So BO1O3=β\angle BO_1O_3 = \beta. Similarly CO3O2=γ\angle CO_3O_2 = \gamma. Hence BO3O1=CO2O3=γ\angle BO_3O_1 = \angle CO_2O_3 = \gamma and BO1O2=CO3O2=β\angle BO_1O_2 = \angle CO_3O_2 = \beta. Thus, CO2O2BO1O3\angle CO_2O_2 \sim \angle BO_1O_3.

Let us denote the radii of ω1\omega_1 by r1r_1, the radii of ω2\omega_2 by r2r_2 and the radii of ω3\omega_3 (the circumcircle of ABC\triangle ABC) by r3r_3. Then r3=CO3=BO3=AO3=ADr_3 = CO_3 = BO_3 = AO_3 = AD, r1=O1B=O1Ar_1 = O_1B = O_1A and r2=O2C=O2Ar_2 = O_2C = O_2A. Also O1BO3=O1AO3\angle O_1BO_3 = \angle O_1AO_3 and O2CO3=O2AO3\angle O_2CO_3 = \angle O_2AO_3. It follows that O1AO3=O2AO3\angle O_1AO_3 = \angle O_2AO_3, which means O1AD=O2AD\angle O_1AD = \angle O_2AD. From CO3O2BO1O3\angle CO_3O_2 \sim \angle BO_1O_3, we have O1BO3B=CO3CO2\frac{O_1B}{O_3B} = \frac{CO_3}{CO_2}, i.e r1r3=r3r2\frac{r_1}{r_3} = \frac{r_3}{r_2}. Then AO1AD=ADAO2\frac{AO_1}{AD} = \frac{AD}{AO_2} holds. So O1ADDAO2\angle O_1AD \sim \angle DAO_2. Hence

DO1DO2=AO1AD=r1r3() \frac{DO_1}{DO_2} = \frac{AO_1}{AD} = \frac{r_1}{r_3} \quad (*)
r1r3=AO1AD=sinADO1sinAO1D=sinADO1sinADO2() \frac{r_1}{r_3} = \frac{AO_1}{AD} = \frac{\sin \angle ADO_1}{\sin \angle AO_1D} = \frac{\sin \angle ADO_1}{\sin \angle ADO_2} \quad (**)
Now we use the law of sines to O2MD\triangle O_2MD and DMO1\triangle DMO_1.
O2MsinO2DM=DO2sinO2MD,O1MsinO1DM=DO1sinO1MD \frac{O_2M}{\sin \angle O_2DM} = \frac{DO_2}{\sin \angle O_2MD}, \quad \frac{O_1M}{\sin \angle O_1DM} = \frac{DO_1}{\sin \angle O_1MD}
From thus, we'll get DO1DO2=sinO2DMsinO1DM\frac{DO_1}{DO_2} = \frac{\sin \angle O_2DM}{\sin \angle O_1DM}. Then from () and (*),
DO1DO2=r1r3=sinADO1sinADO2,sinO2DMsinO1DM=sinADO1sinADO2 \frac{DO_1}{DO_2} = \frac{r_1}{r_3} = \frac{\sin \angle ADO_1}{\sin \angle ADO_2}, \quad \frac{\sin \angle O_2DM}{\sin \angle O_1DM} = \frac{\sin \angle ADO_1}{\sin \angle ADO_2}
it is equivalent to
sin(O1DO2O1DM)sinO1DM=sin(O1DO2ADO2)sinADO2 \frac{\sin(\angle O_1 DO_2 - \angle O_1 DM)}{\sin \angle O_1 DM} = \frac{\sin(\angle O_1 DO_2 - \angle ADO_2)}{\sin \angle ADO_2}
sinO1DO2cosO1DMcosO1DO2sinO1DMsinO1DM=sinO1DO2cosADO2cosO1DO2sinADO2sinADO2 \frac{\sin \angle O_1 DO_2 \cos \angle O_1 DM - \cos \angle O_1 DO_2 \sin \angle O_1 DM}{\sin \angle O_1 DM} = \frac{\sin \angle O_1 DO_2 \cos \angle ADO_2 - \cos \angle O_1 DO_2 \sin \angle ADO_2}{\sin \angle ADO_2}
sinO1DO2ctgO1DMcosO1DO2=sinO1DO2ctgADO2cosO1DO2 \sin \angle O_1 DO_2 \operatorname{ctg} \angle O_1 DM - \cos \angle O_1 DO_2 = \sin \angle O_1 DO_2 \operatorname{ctg} \angle ADO_2 - \cos \angle O_1 DO_2
Since sinO1DO20\sin \angle O_1 DO_2 \neq 0, we obtain ctgO1DM=ctgADO2\operatorname{ctg} \angle O_1 DM = \operatorname{ctg} \angle ADO_2. Hence O1DM=ADO2\angle O_1 DM = \angle ADO_2 (0<O1DM,ADO2<1800^\circ < \angle O_1 DM, \angle ADO_2 < 180^\circ).

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