Borders Investigation

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Borders Investigation

Introduction

============

The starting point for this investigation is the following sequence of

cross shapes:

[IMAGE]

The sequence begins with a single white square, which is then

surrounded by black squares to form the second shape. Each new cross

is then formed by completely surrounding the previous cross with a

border of black squares. In each new cross, the previous cross can be

seen as the area of white squares in the centre.

The aim of this investigation is to derive algebraic formulae from the

sequence, each expressing one property in terms of another (e.g.

defining the area as the diameter squared). These formulae can then be

examined and, hopefully, proven, using a wide variety of mathematical

tools.

The next stage will then be to apply this same process to the

three-dimensional counterpart of this sequence of shapes. This

sequence would begin with a single white cube, which would then be

surrounded on all six sides by black cubes, and the sequence would

continue in the same way as the two-dimensional version.

2D

==

Defining n

Throughout this investigation, the variable n (the value in the phrase

"nth term") is used to refer to terms in sequences. Terms are compared

to, and expressed as functions of, n. However, in order to be able to

identify the value of n for a random shape, we must define what the

first term is in each sequence, and relate n to a physical property of

the shape.

For our 2D cross, I will define the first cross as a single square,

but give it a value of n equal to zero. This means that the value of n

for any 2D cross shape can be seen as the number of borders that

surround the centre square, or the number of squares which the shape

extends away from the centre.

[IMAGE]

[IMAGE]

The diameter can now be conveniently expressed as [IMAGE]:

Perimeter

If we measure the perimeter of the first four crosses and put them on

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