# Fun Topics in Elementary Geometry

Here is a collection of fun topics more or less related to things we've done in class. Enjoy!

## Logo drawing of a 9-pointed star ¶

We used the walking-and-turning method to observe that a person
who walks around the edges of a convex polygon makes exactly 360°
of rotation. With this knowledge, we calculated the interior angles
in a regular n-gon for various n. But we can use a related idea to
understand the interior angles in a star shape. The key idea is that
if you were to walk around the edges of a star, you would make
*two* full turns! The total amount of turning is the sum of
the exterior angles, so for a regular 9-pointed star, each exterior
angle is 80° (80° = 720° ÷ 9). This means each
interior angle is 100°. Below is a drawing, using the Pappert
Logo emulator:

Using the same reasoning, it's easy to make a 13-pointed star, or
to make a star which involves *three* full revolutions -- try
it out!

## Venn diagram of quadrilaterals, using the same quadrilaterals ¶

At lunch one day, I was telling some friends about our class discussion of drawing a Venn diagram showing several different types of quadrilaterals: trapezoids, parllelograms, kites, rhombuses, rectangles, and squares.

Someone remarked that I could make the problem even harder by requiring that you use an example of each shape to represent the set of those shapes! This was mostly a frivolous suggestion, but I realized there's something just slightly interesting about it: can you get the intersection of a rhombus and a rectangle to be a square? And then I drew this picture. For fun, I constructed all of the shapes using the compass and straight-edge techniques we've also learned in class.

Note that there is a boring way to do this too: you could use a square for each shape, since a square satisfies the definition of every one of the quadrilaterals on our list. The drawing above is more interesting, I think :)

## The moduli space of quadrilaterals ¶

A moduli space for a bunch of things is an object with the
property that every *point* on that object corresponds to one
of the things, and *moving through* the space gives you a
continuously changing family of the things. For example, the moduli
space of squares is the line \((0,\infty)\): every point on the line
corresponds to a square (with that side length), and moving along the
line moves through all possible squares.

Now can you visualize the moduli spaces for other quadrilaterals?
If so, can you see how they *fit together* !? Here are
pictures showing the moduli spaces of parallelograms and trapezoids!

### Parallelograms

Above: Parallelograms with side lengths *b* ≤ *a*, and
interior angle 0° < *phi* ≤ 90°. The green face,
where *phi* = 90° contains all rectangles. The
blue face, where *a *= *b*, contains all
rhombuses. The squares lie on their intersection.

### Trapezoids

Above: Trapezoids with a fixed side lengths *a* and *b* = 1,
interior angle 0° < *phi* ≤ 90°,
and exterior angle *phi + delta* where
0° ≤ *delta* ≤ 180° - 2**phi*.
Note that the side length *a* is constrained by the cosine of
*phi*. The curved gray surface is where trapezoids with small
*a* become triangles. The yellow plane, where
*phi*+*delta* = 180° - *phi*
contains isosceles trapezoids. The red plane, where
*delta* = 0° and
*a* ≥ *b* = 1, contains the
parallelograms, and is the same red plane shown in the previous
pictures. The purple line is where *a* = 1.

## A hypercube ¶

The hypercube is a 4-dimensional shape constructed similarly to the
way a cube is constructed. In fact, if you imagine making a cube by
pushing a square in the direction of *the third dimension*,
then you understand how to make a hypercube--just push the cube
in *the fourth dimension*.

This is the natural extension of our understanding of one, two, and three dimensions, and also comes up if you spend a while thinking about the moduli problem above. Below is a picture of the hypercube; follow the link for an animated gif which demonstrates pushing into dimensions 1, 2, 3, and 4.

This image and the accompanying animated gif were created with the open source mathematics software Sage. Here is a Sage worksheet file which generates these images.

For further reading, I recommend Tom Banchoff's essay on Drawing Cubes, part of a series on dimension. He describes different ways of drawing a cube, and at the end discusses how to use those ideas to draw a hypercube in various positions, with pictures!

## Big, BIG numbers ¶

1 00000 00000 00000 00000 00000 00000 00000 00000 00000 00000 00000 00000 00000 00000 00000 00000 00000 00000 00000 00000

That's a googol, 10^100. It's a number larger than the number of atoms in the observable universe–way larger.

A googolplex is 10^(10^100). To write down all
the digits in a googolplex, you would have to write down a one
followed by 10^100 zeroes. In class we calculated how long this would
take: it takes about 45 seconds to write down 100 zeroes. At that
rate, it would take about 450000000 seconds to write down a billion
zeroes -- that's 14.3
years. So the number of years to write down all the digits in a
googolplex would be 1.4 × 10^(91). For contrast, the age of the
universe is estimated to be 13.75 billion
years, which is about 1.4 × 10^7 years. That's
*tiny* compared with the numbers we've been talking about.

Now of course it's easy to imagine bigger and bigger numbers
by taking more powers of 10. It's not so easy to imagine bigger and bigger prime numbers.
Well, of course there are infinitely many, so it's clear that bigger
and bigger primes exist, but it's not so easy to think of a pattern
for producing them. In fact, it's *extremely difficult*, and
is related to a lot of interesting research in mathematics.

As of 2008, the largest known prime is 2^(43112609)-1. That's around 3.2 × 10^(10^7), which makes it more than a googol, but less than a googolplex. Just verifying the number is prime requires clever theoretical research, specialized computer programs, and many days on a supercomputer.

## Digits of pi ¶

Since all circles are similar, the ratio
`circumference`/`diameter` is a constant, the same for
every circle, *ever*. This number, being such a fundamental
property of circles, is of course famous. It's usually represented by
the Greek letter π, pronounced "pi".

Knowing this number is powerful knowledge then. If you know the diameter of a circle, and this number π, then you can calculate the circumference by multiplying. We saw in class that there are easy ways to get simple estimates for π. The picture at right, for example, shows that π is between 3 and 4. If you only knew 1 significant digit of π, then you would be able to calculate the circumference of a circle from its diameter accurately to 1 significant digit. Realistically, this is good enough for many purposes. The more digits of π you know, the more accurate your calculation would be.

Suppose you wanted to calculate the circumference of a circle whose diameter is 4 × 10^25 meters (the approximate diameter of the observable universe). How many digits of π would you need to know in order to have error less than 10^(-12) meters (the approximate width of a hydrogen atom)? Well, 25 + 12 is 37. Since we're multiplying by 4, the next two digits can also contribute to the 37th digit of our answer, so 39 digits of π would suffice! Surprisingly few, don't you think? (Of course, you'd also need as many significant digits in your diameter measurement!) Here are the first 40 digits of π:

3. 14159 26535 89793 23846 26433 83279 50288 4197

## Volume of a sphere ¶

Archimedes calculated the volume of a sphere by comparing it with
a cylinder and two cones. Compare the slice at height *h*
inside the sphere with the annulus at height *h* lying between
the cones and the cylinder. Using the Pythagorean theorem one can see
that each pair of slices have the same area, and therefore the volume
of the sphere is equal to the difference between the volume of the
cylinder and that of the two cones! This explains why the volume
formula for the sphere has a factor of 4/3.
from

The animation below shows these slices.

## Parallel lines and nonzero curvature ¶

The parallel postulate is a description of how parallel lines
behave on a plane -- that is, on a 2-dimensional flat space. In class
we mentioned that straight lines on a curved space can behave
differently. On a sphere, for instance, *every* pair of
straight lines cross somewhere. The straight lines are circles on the
sphere which, if they were elastic, wouldn't naturally shrink to
smaller circles. On the surface of the Earth, something like the
arctic circle would not be a straight line, because it would shrink up
to the North pole. Something like the equator or a line of longitude
would be straight, because trying to tighten them wouldn't cause them
to move at all.

The sphere is a surface with what's known as "positive curvature", while a flat plane has "zero curvature". In class, we talked about the hyperbolic plane -- it has what's known as "negative curvature". Once you're looking for it, you'll notice negative curvature in all kinds of ordinary places. Certain kinds of coral have it, and some fish. Also some cacti, and cooked bacon, and leaves of green lettuce.

tags: fun-topics