$1.98 Mathematics, Part 2

A couple years ago, I was walking about a 99-cent store and found a quad-ruled composition notebook and a box of colored pencils. Total: $1.98. I played around with them for a while, and then they were put in a draw and forgotten about. Until recently when I found the notebook. And then last week, I posted a sketch for those pages.

Here is another one:

This was actually the first sketch from the book, but is wasn't as colorful as the other one. And it seemed to be more boring. But is it really?

The sketch (and you can click on the image for a larger version) shows a Golden Spiral created by connecting the diagonals of adjoining squares. The length of the sides of each square are determined by using the next number in the Fibonacci sequence. Obviously, the squares increased in size so quickly that I couldn't finish the 34 x 34 square.

But there was something else I noticed. I had added extra diagonals to some of the rectangles that were created in addition to the squares. I highlighted one of them in red (on the scan -- it's in pencil on the original sketch). The red line appears to be the diagonal for many of the rectangles. Four of them, in fact.

How could that possibly be the case?
(If any of my students are reading this, STOP here and look at it. Investigate. See if you can figure it out. Come back when you have it or you've had enough. I'll wait.)

The four rectangles have the following sizes: 2 x 1, 5 x 3, 13 x 8 and 34 x 21. Zooming in shows that the red line really isn't a diagonal of the smallest rectangle, so let's discard that one for a moment. The others are close enough to be errors in sketching. Since the slope of a straight line has to be constant, if we calculate the slope at any two points, we should get the same number.

Slope can be calculated as rise over run or change in vertical over change in horizontal. (You remember that "delta y / delta x" thing I keep mentioning in class? Yeah, that.)

So we have slopes of 3/5, 8/13, and 21/34, which are definitely not equivalent fractions. (How do we know that?)

If we convert those fractions to decimals, look what we get:3/5 = 0.6
8/13 = 0.615384...
21/34 = 0.617647...
and if the paper had been bigger, we might have seen
55/89 = 0.6179775...
144/233 = 0.61802565...

So the slopes are nearly identical meaning that the diagonal of the big rectangle isn't really the diagonal of the others, but it's really, really close.

Extra points if anyone keeps going, or if they can tell me the significance of a particular number that starts 0.61803...