Maths Olympiad Prep

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Geometry Difficulty 3.7 AMC 10/12 Find the answer Canada

In the diagram, OO is the
centre of a circle with radius 8787,
and PP and MM lie on the circle. NN is positioned inside the circle so that
PNPN passes through OO and is perpendicular to MNMN.

If MN=63MN=63, what is the area of
PMN\triangle PMN?

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.