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Geometry Difficulty 6.6 National olympiad Prove it

1. (AUS 2) IMO2{ }^{\mathrm{IMO} 2} Let ABCA B C be a triangle. The bisector of angle AA meets the circumcircle of triangle ABCA B C in A1A_{1}. Points B1B_{1} and C1C_{1} are defined similarly. Let AA1A A_{1} meet the lines that bisect the two external angles at BB and CC in point A0A^{0}. Define B0B^{0} and C0C^{0} similarly. If SX1X2XnS_{X_{1} X_{2} \ldots X_{n}} denotes the area of the polygon X1X2XnX_{1} X_{2} \ldots X_{n}, prove that SA0B0C0=2SAC1BA1CB14SABC. S_{A^{0} B^{0} C^{0}}=2 S_{A C_{1} B A_{1} C B_{1}} \geq 4 S_{A B C} .

Solution

1. Let II denote the intersection of the three internal bisectors. Then IA1=A1A0I A_{1}=A_{1} A^{0}. One way proving this is to realize that the circumcircle of ABCA B C is the nine-point circle of A0B0C0A^{0} B^{0} C^{0}, hence it bisects IA0I A^{0}, since II is the orthocenter of A0B0C0A^{0} B^{0} C^{0}. Another way is through noting that IA1=A1BI A_{1}=A_{1} B, which follows from A1IB=IBA1=(A+B)/2\angle A_{1} I B=\angle I B A_{1}=(\angle A+\angle B) / 2, and A1B=A1A0A_{1} B=A_{1} A^{0} which follows from A1A0B=A1BA0=90IBA1\angle A_{1} A^{0} B=\angle A_{1} B A^{0}=90^{\circ}-\angle I B A_{1}. Hence, we obtain SIA1B=SA0A1BS_{I A_{1} B}=S_{A^{0} A_{1} B}. Repeating this argument for the six triangles that have a vertex at II and adding them up gives us SA0B0C0=2SAC1BA1CB1S_{A^{0} B^{0} C^{0}}=2 S_{A C_{1} B A_{1} C B_{1}}. To prove SAC1BA1CB12SABCS_{A C_{1} B A_{1} C B_{1}} \geq 2 S_{A B C}, draw the three altitudes in triangle ABCA B C intersecting in HH. Let X,YX, Y, and ZZ be the symmetric points of HH with respect to the sides BC,CAB C, C A, and ABA B, respectively. Then X,Y,ZX, Y, Z are points on the circumcircle of ABC\triangle A B C (because BXC=BHC=180A)\left.\angle B X C=\angle B H C=180^{\circ}-\angle A\right). Since A1A_{1} is the midpoint of the arc BCB C, we have SBA1CSBXCS_{B A_{1} C} \geq S_{B X C}. Hence
SAC1BA1CB1SAZBXCY=2(SBHC+SCHA+SAHB)=2SABC S_{A C_{1} B A_{1} C B_{1}} \geq S_{A Z B X C Y}=2\left(S_{B H C}+S_{C H A}+S_{A H B}\right)=2 S_{A B C}

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.