Thursday, October 10, 2019

Geometry Puzzle

How I can draw 30 equally spaced points on a circle? This was my initial reaction to the puzzle.

I then decided to experiment with other numbers that are easier to plot to find a pattern.

So, realizing that it's much simpler to divide everything into halves and plotting the points that way, I went on to use 2, 4, 8, and then 16 equally spaced points, drew them on a piece of paper using a Starbucks cup and labelled each point clockwise (as shown in photo).



I noticed that for each diagram I have drawn, the pairs of diametrical opposite points look like this:

Number of Points                       Opposite Pairs

Two points                                  point 1 - point 2

Four points                                  point 1 - point 3
                                                    point 2 - point 4

Eight points:                               point 1 - point 5
                                                    point 2 - point 6
                                                    point 3 - point 7
                                                    point 4 - point 8

Sixteen points                             point 1 - point 9
                                                    point 2 - point 10
                                                    point 3 - point 11
                                                    point 4 - point 12
                                                    point 5 - point 13
                                                    point 6 - point 14
                                                    point 7 - point 15
                                                    point 8 - point 16

Consequently, I realized that there is a pattern and formula to find the opposite point for each particular point on the circle. Suppose that there are n equally spaced points, then:

Opposite point = Base point + n/2, when base point < n/2; and
Opposite point = Base point - n/2, when base point > n/2

Using this formula, the opposite point for point #7 on a 30-point circle is 7 + 30/2 = 22, so point #22.

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