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Geometry Difficulty 6.9 National Olympiad Prove it Slovenia

Let O1O_1 be the centre of the circle K1K_1. Let K2K_2 be the circle centered at O2O_2 and passing through O1O_1. The circles K1K_1 and K2K_2 meet at AA and BB. The circle K1K_1 intersects the segment O1O2O_1O_2 at CC. The line BCBC intersects the circle K2K_2 at BB and DD. The line ADAD intersects the circle K1K_1 at AA and EE. Let FF be the midpoint of the segment AEAE. Prove that the lines O1AO_1A and O1DO_1D divide the angle CO1FCO_1F into three equal parts.

Solution

Write ABD=α\angle ABD = \alpha. Since AO1BDAO_1BD is a cyclic quadrilateral, we have AO1D=ABD=α\angle AO_1D = \angle ABD = \alpha. The central angle AO1C\angle AO_1C in the circle K1K_1 is twice the size of the inscribed angle ABC\angle ABC, so AO1C=2α\angle AO_1C = 2\alpha.

Figure 1

DO1C=AO1CAO1D=2αα=α. \begin{aligned} \angle DO_1C &= \angle AO_1C - \angle AO_1D \\ &= 2\alpha - \alpha = \alpha. \end{aligned}
The segment ABAB is perpendicular to the segment O1O2O_1O_2 and the quadrilateral AO1BO2AO_1BO_2 is a deltoid. So,
ABO1=O1AB=π2O2O1A=π2AO1DDO1C=π22α. \begin{aligned} \angle ABO_1 &= \angle O_1AB = \frac{\pi}{2} - \angle O_2O_1A \\ &= \frac{\pi}{2} - \angle AO_1D - \angle DO_1C = \frac{\pi}{2} - 2\alpha. \end{aligned}

Since AO1BDAO_1BD is a cyclic quadrilateral, we have ADO1=ABO1=π22α\angle ADO_1 = \angle ABO_1 = \frac{\pi}{2} - 2\alpha. Since FF is the midpoint of the segment AEAE, we have AFO1=π2\angle AFO_1 = \frac{\pi}{2}. This implies FO1D=π2FDO1=2α\angle FO_1D = \frac{\pi}{2} - \angle FDO_1 = 2\alpha and
AO1F=DO1FDO1A=2αα=α. \angle AO_1F = \angle DO_1F - \angle DO_1A = 2\alpha - \alpha = \alpha.
We have shown that AO1F=α=AO1C=DO1C\angle AO_1F = \alpha = \angle AO_1C = \angle DO_1C, so the lines O1AO_1A and O1DO_1D divide the angle CO1FCO_1F into three equal parts.

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