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Geometry Difficulty 3.7 AMC 10/12 Find the answer Canada

Eight-sided polygon ABCDEFGHABCDEFGH has integer side lengths. It can be divided into a rectangle and a square, as shown.Figure 0The area of the square is greater than the area of the rectangle. The product of the two areas is equal to 98. Which of the following could be the perimeter of ABCDEFGHABCDEFGH?

Pick one

Solution

Since ABCDABCD is a square and its side lengths are integers, then its area is equal to a perfect square. Since the product of the areas of ABCDABCD and EFGHEFGH (the rectangle) is equal to 98, then the area of ABCDABCD is a divisor of 98. The positive divisors of 98 are 1, 2, 7, 14, 49, and 98. There are exactly two divisors of 98 that are perfect squares, namely 1 and 49. Since the area of ABCDABCD is greater than the area of EFGHEFGH, then the area of ABCDABCD is 49, and so the area of EFGHEFGH is 2 (since 49×2=9849\times2=98). Square ABCDABCD has area 49, and so AB=BC=CD=DA=7AB=BC=CD=DA=7. [[IMAGE0]] The perimeter of ABCDEFGHABCDEFGH is equal to AB+BC+CD+DE+EF+FG+GH+HA=7+7+7+DE+EF+EH+GH+HA(since EH=FG)=21+DE+EH+HA+EF+GH(reorganizing)=21+DA+EF+GH(since DE+EH+HA=DA)=21+7+EF+GH(since DA=7)=28+2×GH(since EF=GH)\begin{align*} &AB+BC+CD+DE+EF+FG+GH+HA & \\ = &7+7+7+DE+EF+EH+GH+HA & \text{(since $EH=FG$)}\\ = &21+DE+EH+HA+EF+GH & \text{(reorganizing)}\\ = &21+DA+EF+GH & \text{(since $DE+EH+HA=DA$)}\\ = &21+7+EF+GH & \text{(since $DA=7$)}\\ = &28+2\times GH & \text{(since $EF=GH$)}\end{align*} Since the side lengths are integers and the area of EFGHEFGH is 2, then either GH=1GH=1 (and FG=2FG=2), or GH=2GH=2 (and FG=1FG=1). If GH=1GH=1, then the perimeter of ABCDEFGHABCDEFGH is 28+2×1=3028+2\times1=30. Since 30 is not given as a possible answer, then GH=2GH=2 and the perimeter is 28+2×2=3228+2\times2=32.

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Source: CEMC, University of Waterloo, licensed CC-BY-NC-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.