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Geometry Difficulty 6.4 National olympiad Prove it Greece

Let c(O,R)c(O, R) be a circle and A,BA, B two diametrically opposite points. The bisector of the angle AB^OA\hat{B}O intersects the circle c(O,R)c(O, R) at point CC, the circumcircle of the triangle AOBAOB (say (c1)(c_1)) at point KK and the circumcircle of the triangle AOCAOC (say (c2)(c_2)) at point LL. Prove that the point KK is the circumcenter of the triangle AOCAOC and the point LL is the incenter of the triangle AOBAOB.

Solution

Segments OBOB, OCOC are equal, as radii of the circle (cc), and so the triangle OBCOBC is isosceles. Hence:
B^1=C^1=x^(1). \hat{B}_1 = \hat{C}_1 = \hat{x} \qquad (1).
Figure 1
Figure 3
BCBC is the bisector of the angle OBAOBA, and hence
B^1=B^2=x^(2). \hat{B}_1 = \hat{B}_2 = \hat{x} \qquad (2).
The angles B^2\hat{B}_2 and O^1\hat{O}_1 are inscribed in the same circle and correspond to same arc OKOK of the circle (c1)(c_1). Hence
B^2=O^1=x^(3). \hat{B}_2 = \hat{O}_1 = \hat{x} \qquad (3).
Similarly we have KO=KCKO = KC and the triangle KOCKOC is isosceles and hence:
O^2=C^1=x^(4). \hat{O}_2 = \hat{C}_1 = \hat{x} \qquad (4).
From (1)–(4) we conclude that: O^1=O^2=x^\hat{O}_1 = \hat{O}_2 = \hat{x}, i.e. OKOK is the bisector, hence and perpendicular bisector of the isosceles triangle OACOAC. The point KK is the midpoint of the arc OKOK (BKBK is the bisector of OBAOBA). Hence the perpendicular bisector of the chord AOAO of the circle (c1c_1) (which is also side of the isosceles triangle OACOAC), is passing through point KK. It means that KK is the circumcenter of the triangle OACOAC.

From (1), (2), we have: B^2=C^1=x^\hat{B}_2 = \hat{C}_1 = \hat{x} and hence ABOCAB \parallel OC. Therefore OA^B=AO^CO\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 in the circle (c2c_2) and correspond to the same arc OLOL. Hence
A^1=C^1=x^. \hat{A}_1 = \hat{C}_1 = \hat{x}.
From the last two equalities we have that A^1=A^2\hat{A}_1 = \hat{A}_2, that is ALAL is the bisector of the angle BA^OB\hat{A}O.

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