Maths Olympiad Prep

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Geometry Difficulty 5.0 AIME Prove it United States

Problem:

Let PP be a point on segment BCBC of triangle ABC\triangle ABC. Let O1O_1 and O2O_2 be the respective circumcenters of ABP\triangle ABP and ACP\triangle ACP. Given BC=O1O2BC = O_1O_2, show that some angle of ABC\triangle ABC is more than 7575^\circ.

Solution

Solution:

WLOG, assume O1O_1 is at least as close to BCBC as is O2O_2. Let M,NM, N, and QQ be the respective midpoints of BP\overline{BP}, CP\overline{CP}, and AP\overline{AP}, and let TT be the foot of the altitude from O1O_1 to O2N\overline{O_2N}.

As O1M\overline{O_1M} and O2N\overline{O_2N} are the perpendicular bisectors of BP\overline{BP} and PC\overline{PC}, respectively, O1TNMO_1TNM is a rectangle, so

O1T=MN=MP+PN=BP2+CP2=BC2=O1O22, O_1T = MN = MP + PN = \frac{BP}{2} + \frac{CP}{2} = \frac{BC}{2} = \frac{O_1O_2}{2},
and O2O1T\triangle O_2O_1T is a respective 3030^\circ-6060^\circ-9090^\circ triangle. Therefore, looking at the sum of the angles of quadrilateral NPQO2NPQO_2, ACP\triangle ACP, and ABC\triangle ABC, we find

- NPQ=360PQO2QO2NO2NP=360903090=150\angle NPQ = 360^\circ - \angle PQO_2 - \angle QO_2N - \angle O_2NP = 360^\circ - 90^\circ - 30^\circ - 90^\circ = 150^\circ,

- ACP=180CPAPAC<180CPA=180150=30\angle ACP = 180^\circ - \angle CPA - \angle PAC < 180^\circ - \angle CPA = 180^\circ - 150^\circ = 30^\circ, and

- A+B=180C>18030=150\angle A + \angle B = 180^\circ - \angle C > 180^\circ - 30^\circ = 150^\circ.

This is impossible unless either A\angle A or B\angle B is

Figure 1

>75>75^\circ, as desired.

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