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Geometry Difficulty 8.5 Shortlist Prove it Baltic Way

Let ω1\omega_1 and ω2\omega_2 be two circles with centers O1O_1 and O2O_2, respectively, with O2O_2 lying on ω1\omega_1. Let AA be a common point of ω1\omega_1 and ω2\omega_2. A line through AA intersects ω1\omega_1 in BAB \neq A and ω2\omega_2 in CAC \neq A such that AA lies between BB and CC. The ray O2O1O_2O_1 intersects ω2\omega_2 in DD and contains a point EE such that EAD=DCO2\angle EAD = \angle DCO_2 and DD lies between O2O_2 and EE. Show that BO2BO_2 bisects CECE.

Solutions — 2

Solution 1

Let FF be the second intersection of ω1\omega_1 and ω2\omega_2. Notice that
BFO2=180BAO2=CAO2=O2CA \angle BFO_2 = 180^\circ - \angle BAO_2 = \angle CAO_2 = \angle O_2CA
and since AO2=EO2AO_2 = EO_2 that FBO2=O2BA\angle FBO_2 = \angle O_2BA. It follows that FF is the reflection of CC over BO2BO_2, thus it suffices to prove that EFEF is parallel to BO2BO_2, as then BO2BO_2 is a midline of triangle CEFCEF.

Notice that O2DC=DCO2=EAD\angle O_2DC = \angle DCO_2 = \angle EAD. Hence by symmetry about O1O2O_1O_2, we have
FEO2=AED=180EADADE=ADO2CDO2=ADC=12AO2C \begin{align*} \angle FEO_2 &= \angle AED = 180^\circ - \angle EAD - \angle ADE \\ &= \angle ADO_2 - \angle CDO_2 = \angle ADC = \frac{1}{2} \angle AO_2C \end{align*}

Moreover
90O1O2B=12BO1O2=180BAO2=CAO2=9012AO2C \begin{aligned} 90^\circ - \angle O_1O_2B &= \frac{1}{2}\angle BO_1O_2 = 180^\circ - \angle BAO_2 \\ &= \angle CAO_2 = 90^\circ - \frac{1}{2}\angle AO_2C \end{aligned}
so we conclude that FEO2=EO2B\angle FEO_2 = \angle EO_2B which proves that EFEF and BO2BO_2 are parallel.

Figure 1

Solution 2

Let GG be the point on ω2\omega_2 diametrically opposite CC. As in the first solution, it suffices to prove that EGEG is parallel to BO2BO_2, as then BO2BO_2 is a midline of triangle CEGCEG. Note first that
GO2E=2ECD=DAE+GAD=GAE \angle GO_2E = 2\angle ECD = \angle DAE + \angle GAD = \angle GAE
so the points AA, EE, GG, and O2O_2 lie on a circle. Since GO2=AO2GO_2 = AO_2, we see that O2O_2 is the midpoint of the arc AGAG of this circle. It follows then that
GEO2=AGO2=12AO2C \angle GEO_2 = AGO_2 = \frac{1}{2}\angle AO_2C
and as we showed in the first solution this implies that GEO2=EO2B\angle GEO_2 = \angle EO_2B, so lines EGEG and BO2BO_2 are parallel.

Figure 2

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