Maths Olympiad Prep

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, 2022

Geometry Difficulty 5.0 AIME Find the answer United States

Problem:

How many ways are there to cut a 11 by 11 square into 88 congruent polygonal pieces such that all of the interior angles for each piece are either 4545 or 9090 degrees? Two ways are considered distinct if they require cutting the square in different locations. In particular, rotations and reflections are considered distinct.

Proposed by: Freddie Zhao

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Solution:

First note that only triangles and quadrilaterals are possible.
There are 33 possibilities:
- 1/21/2 by 1/21/2 right isosceles triangles
- 11 by 1/81/8 rectangles
- 1/21/2 by 1/41/4 rectangles

The first case has 1616 possibilities (there are 22 choices for the orientation of each quadrant).
The second case has 22 possibilities (either all horizontal or all vertical).

The third case is the trickiest. Label the quadrants A,B,C,DA, B, C, D where A,BA, B are at the top and B,CB, C are on the left. If each rectangle lies completely within a quadrant, there are 1616 ways. If rectangles span quadrants A,BA, B but not CC or DD, there are 44 ways. Similarly, there are 44 ways each for [rectangles spanning B,CB, C but not D,AD, A], [rectangles spanning C,DC, D but not A,BA, B], and [rectangles spanning D,AD, A but not B,CB, C]. Next, if rectangles span both A,BA, B and C,DC, D, there is 11 way, and if rectangles span both B,CB, C and D,AD, A there is 11 way. Finally there are 22 ways for each adjacent pair of quadrants to have a rectangle spanning them. This brings us to 16+4+4+4+4+1+1+2=3616+4+4+4+4+1+1+2=36 ways.

The final answer is 16+2+36=5416+2+36=54.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.