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Geometry Difficulty 6.2 National Olympiad Prove it JBMO

Problem:

Let cc(O,K)c \equiv c(O, K) be a circle with center OO and radius RR and A,BA, B be two points on it, not belonging to the same diameter. The bisector of the angle AB^OA \hat{B} O intersects the circle cc at point CC, the circumcircle of the triangle AOBA O B, say c1^{c_{1}} at point KK and the circumcircle of the triangle AOCA O C, say c2^{c_{2}}, at point LL. Prove that the point KK is the circumcenter of the triangle AOCA O C and the point LL is the incenter of the triangle AOBA O B.

Solution

Solution:

The segments OB,OCO B, O C are equal, as radii of the circle c^{c}. Hence OBCO B C is an isosceles triangle and
B^1=C^1=x^ \hat{B}_{1}=\hat{C}_{1}=\hat{x}

Figure 1

The chord BCB C is the bisector of the angle OB^AO \hat{B} A, and hence
B^1=B^2=x^ \hat{B}_{1}=\hat{B}_{2}=\hat{x}
The angles B^2\hat{B}_{2} and O^1\hat{O}_{1} are inscribed to the same arc OKO K of the circle c1^{c_{1}} and hence
B^2=O^1=x^ \hat{B}_{2}=\hat{O}_{1}=\hat{x}

The segments KO,KCK O, K C are equal, as radii of the circle c2^{c_{2}}. Hence the triangle KOCK O C is isosceles and so
O^2=C^1=x^ \hat{O}_{2}=\hat{C}_{1}=\hat{x}
From equalities (1),(2),(3)(1),(2),(3) we conclude that
O^1=O^2=x^ \hat{O}_{1}=\hat{O}_{2}=\hat{x}
and so OKO K is the bisector, and hence perpendicular bisector of the isosceles triangle OACO A C. The point KK is the middle of the arc OKO K (since BKB K bisects the angle OB^AO \hat{B} A). Hence the perpendicular bisector of the chord AOA O of the circle c1^{c_{1}} is passing through point KK. It means that KK is the circumcenter of the triangle OACO A C.

From equalities (1),(2),(3) we conclude that B^2=C^1=x^\hat{B}_{2}=\hat{C}_{1}=\hat{x} and so ABOCOA^B=AO^CA B \parallel O C \Rightarrow O \hat{A} B=A \hat{O} C, that is A^1+A^2=O^1+O^2\hat{A}_{1}+\hat{A}_{2}=\hat{O}_{1}+\hat{O}_{2} and since O^1=O^2=x^\hat{O}_{1}=\hat{O}_{2}=\hat{x}, we conclude that
A^1+A^2=2O^1=2x^ \hat{A}_{1}+\hat{A}_{2}=2 \hat{O}_{1}=2 \hat{x}
The angles A^1\hat{A}_{1} and C^1\hat{C}_{1} are inscribed into the circle c2^{c_{2}} and correspond to the same arc OLO L. Hence
A^1=C^1=x^ \hat{A}_{1}=\hat{C}_{1}=\hat{x}
From (5) and (6) we have A^1=A^2\hat{A}_{1}=\hat{A}_{2}, i.e. ALA L is the bisector of the angle BA^OB \hat{A} O.

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