Maths Olympiad Prep

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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}=ANDE\{P\} = AN \cap DE and {Q}=AMEF\{Q\} = AM \cap EF.

Figure 1

Solution

Because ABMADN\triangle ABM \equiv \triangle ADN and DAEDCF\triangle DAE \equiv \triangle DCF, we have m(BAM)=m(DAN)=m(ADE)=m(CDF)=15m(\overline{BAM}) = m(\overline{DAN}) = m(\overline{ADE}) = m(\overline{CDF}) = 15^\circ.
Then ABMADNDAEDCF\triangle ABM \equiv \triangle ADN \equiv \triangle DAE \equiv \triangle DCF and AMNDEF\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 PFDEPF \perp DE.
As m(DAM)+m(ADE)=m(DAM)+m(BAM)=90m(\overline{DAM}) + m(\overline{ADE}) = m(\overline{DAM}) + m(\overline{BAM}) = 90^\circ, we get that AMDEAM \perp DE, obtaining that AMPFAM \parallel PF.
The equality m(PAQ)=m(PEQ)=60m(\overline{PAQ}) = m(\overline{PEQ}) = 60^\circ implies that PAEQPAEQ is a cyclic quadrilateral, and thus m(FPQ)=m(EPF)m(EPQ)=75m(\overline{FPQ}) = m(\overline{EPF}) - m(\overline{EPQ}) = 75^\circ.
On the other hand, m(MFP)=180m(CFD)m(DFP)=75m(\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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