Maths Olympiad Prep

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Geometry Difficulty 5.4 AIME, harder Prove it Italy

Problem:

Lucio buys a table shaped like a regular hexagon and a rectangular tablecloth of area 1024 cm21024~\mathrm{cm}^2 whose shorter side equals the side of the hexagon and which covers exactly the portion of the table between two opposite sides. What is the area of the table, expressed in cm2\mathrm{cm}^2?

Solution

Solution:

The answer is 1536.

Figure 1

Let ABCDEFA B C D E F be a regular hexagon representing the table; the tablecloth of area 1024 cm21024~\mathrm{cm}^2 covers exactly the rectangle ABDEA B D E. Letting OO be the center of the hexagon and H,KH, K be the points of intersection between the segments BDB D and OCO C, AEA E and OFO F, respectively, the triangles OAKO A K, OEKO E K, OBHO B H and ODHO D H are all congruent and have an area equal to half the area of the equilateral triangles ABOA B O and DEOD E O. It follows that the area of the rectangle ABDEA B D E equals four times the area of ABOA B O, while the area of the table equals six times the area of ABOA B O. We conclude that the area of the table is 641024 cm2=1536 cm2\frac{6}{4} \cdot 1024~\mathrm{cm}^2 = 1536~\mathrm{cm}^2.

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