Let ABC be a triangle and P an interior point of it. Let H be the point on side BC such that the bisector of angle AHP is perpendicular to line BC. Knowing that ABC=HPC and BPC=130∘, determine the measure of angle BAC.
Solution
Let P′ be the reflection of point P with respect to line BC. Note that points A, H, P′ are collinear, since AHP′=AHP+2PHC=AHP+2(90∘−AHP/2)=180∘.
Now, AP′C=HP′C=HPC by symmetry, but HPC=ABC by hypothesis; this implies the cyclicity of quadrilateral ABP′C.
This cyclicity implies that BAC is supplementary to BP′C, which is congruent to BPC by symmetry, and hence BAC=180∘−BPC=180∘−130∘=50∘.
Second solution. Let C′ be the intersection of line CP with side AB; quadrilateral BHPC′ is cyclic (since HPC=HBC′ by hypothesis, and thus the opposite angles HBC′ and C′PH are supplementary).
Note moreover that, denoting by 2θ the angle PHA, we have PHC=90∘−θ=C′HB; using again the identity HPC=HBC′ in triangles BAH and PCH, it follows by difference that BAH=PCH=C′CH. It follows that quadrilateral AC′HC is also cyclic.
There are now various ways to conclude using angle identities given by cyclicity; for example, HBP=HC′P by cyclicity of BHPC′, and HC′P=HAC by cyclicity of AC′HC; but then BAC=BAH+HAC=PCB+PBC=180∘−BPC=50∘.
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