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Geometry Difficulty 6.3 National Olympiad Prove it Iran

Points XX and YY are located on sides ABAB and ACAC of triangle ABCABC (X,YAX, Y \neq A) such that the reflection of line BCBC with respect to XYXY is tangent to the circumcircle of triangle AXYAXY. If OO denotes the circumcenter of triangle ABCABC, prove that the circumcircle of triangle AXYAXY is tangent to the circumcircle of triangle BOCBOC.

Solution

Let ω\omega be the circumcircle of triangle AXYAXY. Since the reflection of line BCBC with respect to XYXY is tangent to ω\omega, the reflection of ω\omega with respect to XYXY is tangent to line BCBC at a point that is denoted by TT.

Let PP be the second intersection point of circumcircles of triangles BXTBXT and CYTCYT (other than TT). It is claimed that circumcircles of triangles BOCBOC and AXYAXY are tangent at PP.

First, note that since TT lies on the reflection of ω\omega with respect to XYXY, XTY=XAY=BAC\angle XTY = \angle XAY = \angle BAC. Therefore,
BPC=BPT+CPT=BXT+CYT=360AXTAYT=XAY+XTY=2BAC=BOC. \begin{align*} \angle BPC &= \angle BPT + \angle CPT = \angle BXT + \angle CYT \\ &= 360^\circ - \angle AXT - \angle AYT = \angle XAY + \angle XTY \\ &= 2\angle BAC = \angle BOC. \end{align*}
This implies that PP lies on the circumcircle of triangle BOCBOC. On the other hand,
180BAC=XBT+YCT=XPY. 180^\circ - \angle BAC = \angle XBT + \angle YCT = \angle XPY.
Therefore, PP lies on the circumcircle of triangle AXYAXY. Now it suffices to show that these two circles have the same tangent line at PP, or equivalently, BPX=BCP+XAP\angle BPX = \angle BCP + \angle XAP. Since BCP=TYP\angle BCP = \angle TYP and XAP=XYP\angle XAP = \angle XYP, BCP+XAP=TYP+XYP=XYT\angle BCP + \angle XAP = \angle TYP + \angle XYP = \angle XYT. However, according to the assumptions, XYT=XTB=BPX\angle XYT = \angle XTB = \angle BPX, which completes the proof.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.