Maths Olympiad Prep

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Geometry Difficulty 5.6 AIME, harder Prove it Italy

Let ABCABC be a triangle and PP an interior point of it. Let HH be the point on side BCBC such that the bisector of angle AHP^\widehat{AHP} is perpendicular to line BCBC. Knowing that ABC^=HPC^\widehat{ABC}=\widehat{HPC} and BPC^=130\widehat{BPC}=130^\circ, determine the measure of angle BAC^\widehat{BAC}.

Solution

Let PP' be the reflection of point PP with respect to line BCBC. Note that points AA, HH, PP' are collinear, since AHP^=AHP^+2PHC^=AHP^+2(90AHP^/2)=180\widehat{AHP'}=\widehat{AHP}+2\widehat{PHC}=\widehat{AHP}+2\left(90^\circ-\widehat{AHP}/2\right)=180^\circ.

Now, APC^=HPC^=HPC^\widehat{AP'C}=\widehat{HP'C}=\widehat{HPC} by symmetry, but HPC^=ABC^\widehat{HPC}=\widehat{ABC} by hypothesis; this implies the cyclicity of quadrilateral ABPCABP'C.

This cyclicity implies that BAC^\widehat{BAC} is supplementary to BPC^\widehat{BP'C}, which is congruent to BPC^\widehat{BPC} by symmetry, and hence BAC^=180BPC^=180130=50\widehat{BAC}=180^\circ-\widehat{BPC}=180^\circ-130^\circ=50^\circ.

Second solution. Let CC' be the intersection of line CPCP with side ABAB; quadrilateral BHPCBHP C' is cyclic (since HPC^=HBC^\widehat{HPC}=\widehat{HBC'} by hypothesis, and thus the opposite angles HBC^\widehat{HBC'} and CPH^\widehat{C'PH} are supplementary).

Note moreover that, denoting by 2θ2\theta the angle PHA^\widehat{PHA}, we have PHC^=90θ=CHB^\widehat{PHC}=90^\circ-\theta=\widehat{C'HB}; using again the identity HPC^=HBC^\widehat{HPC}=\widehat{HBC'} in triangles BAHBAH and PCHPCH, it follows by difference that BAH^=PCH^=CCH^\widehat{BAH}=\widehat{PCH}=\widehat{C'CH}. It follows that quadrilateral ACHCAC'H C is also cyclic.

There are now various ways to conclude using angle identities given by cyclicity; for example, HBP^=HCP^\widehat{HBP}=\widehat{HC'P} by cyclicity of BHPCBHPC', and HCP^=HAC^\widehat{HC'P}=\widehat{HAC} by cyclicity of ACHCAC'HC; but then BAC^=BAH^+HAC^=PCB^+PBC^=180BPC^=50\widehat{BAC}=\widehat{BAH}+\widehat{HAC}=\widehat{PCB}+\widehat{PBC}=180^\circ-\widehat{BPC}=50^\circ.

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