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Geometry Difficulty 8.5 Shortlist Prove it Switzerland

Problem:

Let ABCA B C be a triangle where BAC=90\angle B A C=90^{\circ}, with circumcenter OO and incenter II. The angle bisector of BAC\angle B A C intersects the circumcircle of ABCA B C in AA and PP. Let QQ be the projection of PP onto ABA B, and RR be the projection of II onto PQP Q. Prove that ROR O bisects CIC I.

Solution

Solution:

Because OO is the midpoint of BC\overline{B C}, it suffices to show that ROR O is parallel to BIB I, as the result follows by simply looking at a homothety centered at CC.

So first of all by looking at the problem we instantly recognise the point PP as the circumcenter of BICB I C, as this is a well known configuration. So from this we PB=PI|P B|=|P I| and BOP=90\angle B O P=90^{\circ}. Next up we find AQA Q to be parallel to IRI R, as they both are perpendicular to PQP Q. From this we get BIR=IBA=IBC\angle B I R=\angle I B A=\angle I B C. We now have two same angles at BB and II and also PB=PI|P B|=|P I|. This gives us motivation to look at perpendicular bisector of BI\overline{B I}, as it goes through PP and X:=RIBCX:=R I \cap B C. If we can show that ROXR O X is also isosceles, then it'll instantly follow that BOR=OBI\angle B O R=\angle O B I, which then will give us ROR O parallel to BIB I as wanted. So now for the fun stuff; more angle chasing.

We easily calculate APQ=OPB=45\angle A P Q=\angle O P B=45^{\circ}. From this we get that BPQ=OPI\angle B P Q=\angle O P I and thereby that the angle bisector of IPB\angle I P B and OPR\angle O P R coincide. And as the angle bisector of IPB\angle I P B is equal to the perpendicular bisector of BIB I, we have XPR=OPR\angle X P R=\angle O P R. Finally using the cyclic quadrilateral OPRXO P R X we have ORX=OPX=XPR=XOR\angle O R X=\angle O P X=\angle X P R=\angle X O R, which finishes the proof as explained before.

Figure 1

Let SS be the intersection of ICI C and ORO R

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