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Romanian Mathematical Olympiad

Geometry Difficulty 5.9 AIME, harder Prove it Romania

Let ABCDABCD be a square. We take the points E∈(AB)E \in (AB), N∈(CD)N \in (CD) and F,M∈(BC)F, M \in (BC), such that the triangles AMNAMN and DEFDEF are equilateral. Prove that PQ=FMPQ = FM, where {P}=AN∩DE\{P\} = AN \cap DE and {Q}=AM∩EF\{Q\} = AM \cap EF.

Figure 1

Solution

Because △ABM≡△ADN\triangle ABM \equiv \triangle ADN and △DAE≡△DCF\triangle DAE \equiv \triangle DCF, we have m(BAM‾)=m(DAN‾)=m(ADE‾)=m(CDF‾)=15∘m(\overline{BAM}) = m(\overline{DAN}) = m(\overline{ADE}) = m(\overline{CDF}) = 15^\circ.
Then △\triangle ABM ≡△\equiv \triangle ADN ≡△\equiv \triangle DAE ≡△\equiv \triangle DCF and △AMN≡△DEF\triangle AMN \equiv \triangle DEF.
It is clear now that ADNEADNE is a rectangle, and thus PP is the common midpoint of the line segments [AN][AN] and [DE][DE]. Then, in the equilateral triangle DEFDEF we have PF⊥DEPF \perp DE.
As m(DAM‾)+m(ADE‾)=m(DAM‾)+m(BAM‾)=90∘m(\overline{DAM}) + m(\overline{ADE}) = m(\overline{DAM}) + m(\overline{BAM}) = 90^\circ, we get that AM⊥DEAM \perp DE, obtaining that AM∥PFAM \parallel PF.
The equality m(PAQ‾)=m(PEQ‾)=60∘m(\overline{PAQ}) = m(\overline{PEQ}) = 60^\circ implies that PAEQPAEQ is a cyclic quadrilateral, and thus m(FPQ‾)=m(EPF‾)−m(EPQ‾)=75∘m(\overline{FPQ}) = m(\overline{EPF}) - m(\overline{EPQ}) = 75^\circ.
On the other hand, m(MFP‾)=180∘−m(CFD‾)−m(DFP‾)=75∘m(\overline{MFP}) = 180^\circ - m(\overline{CFD}) - m(\overline{DFP}) = 75^\circ, which implies that MQPFMQPF is an isosceles trapezoid and thus [PQ]=[FM][PQ] = [FM].

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