Maths Olympiad Prep

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Geometry Difficulty 5.8 AIME, harder Prove it United States

Problem:

Let ABCDABCD be a quadrilateral circumscribed about a circle with center OO. Let O1,O2,O3O_{1}, O_{2}, O_{3}, and O4O_{4} denote the circumcenters of AOB\triangle AOB, BOC\triangle BOC, COD\triangle COD, and DOA\triangle DOA. If A=120\angle A = 120^{\circ}, B=80\angle B = 80^{\circ}, and C=45\angle C = 45^{\circ}, what is the acute angle formed by the two lines passing through O1O3O_{1}O_{3} and O2O4O_{2}O_{4}?

Solution

Solution:

Answer: 82.582.5

Lemma: Given a triangle ABC\triangle ABC, let II be the incenter, IAI_{A} be the excenter opposite AA, and Sˇ\check{S} be the second intersection of AIAI with the circumcircle. Then Sˇ\check{S} is the center of the circle through B,I,CB, I, C, and IAI_{A}.

Proof. First, note
IBIA=IBC+CBIA=ABC2+180ABC2=90. \angle IBI_{A} = \angle IBC + \angle CBI_{A} = \frac{\angle ABC}{2} + \frac{180^{\circ} - \angle ABC}{2} = 90^{\circ}.
Similarly ICIA=90\angle ICI_{A} = 90^{\circ}. Therefore BICIABICI_{A} is cyclic. Now note that A,I,SˇA, I, \check{S}, and IAI_{A} are collinear because they are all on the angle bisector of BAC\angle BAC. Hence
CISˇ=180CIA=CAI+ACI=BCSˇ+ICB=ICSˇ \angle CI\check{S} = 180^{\circ} - \angle CIA = \angle CAI + \angle ACI = \angle BC\check{S} + \angle ICB = \angle IC\check{S}
(Note CAI=BASˇ=BCSˇ\angle CAI = \angle BA\check{S} = \angle BC\check{S} since A,B,SˇA, B, \check{S}, and CC are concyclic.) Hence SˇC=SˇI\check{S}C = \check{S}I. Similarly SˇB=SˇI\check{S}B = \check{S}I. Thus Sˇ\check{S} is the center of the circle passing through B,IB, I, and CC, and therefore IAI_{A} as well.

Let BABA and CDCD intersect at EE and DADA and CBCB intersect at FF. We first show that F,O1,OF, O_{1}, O, and O3O_{3} are collinear.

Let O1O_{1}' and O3O_{3}' denote the intersections of FOFO with the circumcircles of triangles FABFAB and FDCFDC. Since OO is the excenter of triangle FABFAB, by the lemma O1O_{1}' is the circumcenter of ABO\triangle ABO; since OO is incenter of triangle FDCFDC, by the lemma O3O_{3}' is the circumcenter of DOC\triangle DOC. Hence O1=O1O_{1}' = O_{1} and O3=O3O_{3}' = O_{3}. Thus, points F,O1,OF, O_{1}, O, and O3O_{3} are collinear, and similarly, we have E,O2,OE, O_{2}, O, and O4O_{4} are collinear.

Now BEC=55\angle BEC = 55^{\circ} and DFC=20\angle DFC = 20^{\circ} so considering quadrilateral EOFCEOFC, the angle between O1O3O_{1}O_{3} and O2O4O_{2}O_{4} is
EOF=OEC+OFC+FCE=BEC2+DFC2+FCE=27.5+10+45=82.5. \begin{aligned} \angle EOF & = \angle OEC + \angle OFC + \angle FCE \\ & = \frac{\angle BEC}{2} + \frac{\angle DFC}{2} + \angle FCE \\ & = 27.5^{\circ} + 10^{\circ} + 45^{\circ} = 82.5^{\circ}. \end{aligned}

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