Determine the number of rectangles and hexagons in the given figure.

20262024

Determine the number of rectangles and hexagons in the given figure.

  1. A.

    30, 5

  2. B.

    32, 3

  3. C.

    28, 5

  4. D.

    30, 3

Attempted by 2 students.

Show answer & explanation

Correct answer: A

In a compound figure built from several overlapping elementary regions, classify every candidate rectangle by how many elementary regions it is made of (1-region, 2-region, 3-region, and so on up to the largest combination), count each class separately, then add the classes together.

A hexagon in such a figure is any closed six-vertex boundary you can trace using the figure's labelled points; find every one by systematically tracing candidate six-sided boundaries around the diagram rather than guessing.

Label the figure as shown, with the outer points A-P and the inner points Q-W marking every intersection.

Rectangle class (components)

Rectangles (by label)

Count

1

CVSR, VETS, RSWM and STKW

4

2

CETR, VEKW, RTKM and CVWM

4

3

ACRP, PRMO, EGHT and THIK

4

4

CEKM, AVSP, PSWO, VGHS and SHIW

5

5

AETP, PTKO, CGHR and RHIM

4

6

ACMO and EGIK

2

8

AGHP, PHIO, AVWO and VGIW

4

10

AEKO and CGIM

2

16

AGIO

1

Total rectangles = 4 + 4 + 4 + 5 + 4 + 2 + 4 + 2 + 1 = 30.

Tracing every six-vertex boundary the same way gives these hexagons:

  • CDEKLM

  • CEUKMQ

  • CFHJMQ

  • BEUKNP

  • BFHJNP

That gives 5 distinct hexagons.

Cross-checking against the other pairings shows why each is off:

  • 32 rectangles over-counts by double-reading some of the larger multi-component rectangles (an eight- or ten-component rectangle counted from two overlapping directions), and 3 hexagons under-counts by missing two of the six-vertex boundaries that share edge points with the outer border.

  • 28 rectangles under-counts by dropping an entire four-component class (shapes such as CEKM or VGHS, each built from two adjacent quadrant pieces).

  • 3 hexagons under-counts for the same reason as above - two boundary hexagons sharing edge points with the outer border get missed when the trace isn't carried all the way around the figure.

So the figure contains 30 rectangles and 5 hexagons.

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