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Geometry Difficulty 6.0 National olympiad Prove it Bulgaria

Given is a triangle ABCABC and a circle ω\omega with center II that touches ABAB, ACAC and meets BCBC at XX, YY. The line through II perpendicular to BCBC meets the line through AA parallel to BCBC at ZZ. Show that the circumcircles of XYZ\triangle XYZ and ABC\triangle ABC are tangent to each other.

Solution

Let WW be the midpoint of the major arc BACBAC, A(ABC)A' \in (ABC) be such that AABCAA' \parallel BC, TT be the intersection of the circle with diameter AIAI and (ABC)(ABC) and let ω\omega touch ACAC, ABAB at EE, FF. We claim the two circles touch at TT.

Firstly, observe that Z(AEF)Z \in (AEF), so
ATZ=AIZ=90(α2+γ)=βγ2=β(90α2)=ABCWBC=ABW=ATW, \begin{aligned} \angle ATZ &= \angle AIZ = 90^\circ - \left(\frac{\alpha}{2} + \gamma\right) \\ &= \frac{\beta - \gamma}{2} = \beta - \left(90^\circ - \frac{\alpha}{2}\right) \\ &= \angle ABC - \angle WBC = \angle ABW = \angle ATW, \end{aligned}
hence TT, ZZ, WW are collinear. Let TWBC=PTW \cap BC = P. By spiral similarity and angle bisector theorem we have PBPC=TBTC=BFCE\frac{PB}{PC} = \frac{TB}{TC} = \frac{BF}{CE}, so by converse of Menelaus theorem for ABC\triangle ABC we obtain that EFEF, TZTZ, BCBC are concurrent at PP. Thus, by power of point at PP, we obtain that PXPY=PEPF=PZPTPX \cdot PY = PE \cdot PF = PZ \cdot PT, so XYZTXYZT is cyclic.

Finally, observe that the center of (ZXY)(ZXY) lies on IZIZ, so (ZXY)(ZXY) touches AAAA' at ZZ. Hence, by shooting lemma, since WW is the midpoint of the minor arc AAAA', we obtain that (ZXY)(ZXY) touches (ABC)(ABC) at TT and we are done. \square

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