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Geometry Difficulty 4.9 AIME Prove it Taiwan

ABC\triangle ABC is an acute triangle. Through a point LL on BCBC, draw a circle ω\omega that is tangent to ABAB at BB' and tangent to ACAC at CC'. Furthermore, suppose that the circumcenter OO of ABC\triangle ABC lies on the shorter arc BCB'C' of ω\omega. Prove that the circumcircle of ABC\triangle ABC intersects ω\omega at two points.

Solution

Clearly BB' is the foot of the perpendicular from LL to ABAB, so it lies within segment ABAB. Similarly, CC' lies within segment ACAC. Therefore OO must lie inside ABC\triangle AB'C', and hence COB<COB\angle COB < \angle C'OB'.

Now let α=CAB\alpha = \angle CAB. It is easy to see that COB=2CAB=2α\angle COB = 2\angle CAB = 2\alpha and 2COB=360CLB2\angle C'OB' = 360^\circ - \angle C'LB'. Also, CLB=180CAB=180α\angle C'LB' = 180^\circ - \angle C'AB' = 180^\circ - \alpha. Combining the above, we get
2α=COB<COB=360CLB2=360(180α)2=90+α2, 2\alpha = \angle COB < \angle C'OB' = \frac{360^\circ - \angle C'LB'}{2} = \frac{360^\circ - (180^\circ - \alpha)}{2} = 90^\circ + \frac{\alpha}{2},
so α<60\alpha < 60^\circ.

Finally, let OO' be the reflection of OO across BCBC. Then in the quadrilateral ABOCABO'C, we have
COB+CAB=COB+CAB=2α+α<180, \angle CO'B + \angle CAB = \angle COB + \angle CAB = 2\alpha + \alpha < 180^\circ,
so OO' must lie outside the circumcircle of ABC\triangle ABC. Therefore, the points OO and OO' lie respectively inside and outside the circumcircle of ABC\triangle ABC, so the circle ω\omega must intersect the circumcircle of ABC\triangle ABC at two points.

Figure 1

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Source: MathNet, licensed CC-BY-4.0. Statement translated into English from zh; metadata (topic, difficulty) added by this project.