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Geometry Difficulty 6.2 National Olympiad Prove it Italy

Problem:

In trapezoid ABCDABCD the sides ABAB and CDCD are parallel, and the angles ABC^\widehat{ABC} and BAD^\widehat{BAD} are acute. Prove that it is possible to divide triangle ABCABC into 4 disjoint triangles X1,,X4X_{1}, \ldots, X_{4} and triangle ABDABD into 4 disjoint triangles Y1,,Y4Y_{1}, \ldots, Y_{4} such that the triangles XiX_{i} and YiY_{i} are congruent for every ii.

Solution

Solution:

Let DD^{\prime} be the reflection of DD with respect to ABAB, and let EE be the intersection of line CDCD^{\prime} with line ABAB. Since the angles BAD^,CBA^\widehat{BAD}, \widehat{CBA} are acute, EE lies inside segment ABAB. Moreover, since CC and DD^{\prime} are equidistant from ABAB, we have CE=EDCE = ED^{\prime}.

Through EE draw the lines r1ACr_{1} \parallel AC and r2ADr_{2} \parallel AD^{\prime}, and denote by GG the intersection of r1r_{1} with ADAD^{\prime}, and by FF the intersection of r2r_{2} with ACAC; by construction we get
AEF=EAG,CEF=EDG. AEF = EAG, \quad CEF = ED^{\prime}G.
Figure 1

Similarly, through EE draw the lines r3BDr_{3} \parallel BD^{\prime} and r4BCr_{4} \parallel BC, and denote by MM the intersection of r3r_{3} with BCBC, and by HH the intersection of r4r_{4} with BDBD^{\prime}; we get
CEM=EDH,BEM=EBH. CEM = ED^{\prime}H, \quad BEM = EBH.

Finally, let O,NO, N be the reflections of H,GH, G with respect to ABAB, and the choice
X1=AEF,X2=CEF,X3=CEM,X4=BEMY1=EAN,Y2=EDN,Y3=EDO,Y4=EBO \begin{gathered} X_{1} = AEF, \quad X_{2} = CEF, \quad X_{3} = CEM, \quad X_{4} = BEM \\ Y_{1} = EAN, \quad Y_{2} = EDN, \quad Y_{3} = EDO, \quad Y_{4} = EBO \end{gathered}
verifies the claim.

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