Hooray, I have finally finished what used to be called EGADs and is now the first volume of *The Model Shop*. Calling it the first volume is, of course, a treacherous decision.

So. This is a book of 42 activities that connect geometry to functions through data. There are a lot of different ways to describe it, and in the course of finishing the book, the emotional roller-coaster took me from great pride in what a great idea this was to despair over how incredibly stupid I’ve been.

I’m obviously too close to the project.

For an idea of what drove some of the book, check out the posts on the “Chord Star.”

But you can also see the basic idea in the book cover. See the spiral made of triangles? Imagine measuring the hypotenuses of those triangles, and plotting the lengths as a function of “triangle number.” That’s the graph you see. What’s a good function for modeling that data?

If we’re experienced in these things, we say, oh, it’s exponential, and the base of the exponent is the square root of 2. But if we’re less experienced, there are a lot of connections to be made.

We might think it *looks* exponential, and use sliders to fit a curve (for example, in Desmos or Fathom. Here is a Desmos document with the data you can play with!) and discover that the base is close to 1.4. Why should it be 1.4? Maybe we notice that if we skip a triangle, the size seems to double. And that might lead us to think that 2 is involved, and gradually work it out that root 2 will help.

Or we might start geometrically, and reason about similar triangles. And from there gradually come to realize that the *a*/*b* = *c*/*d* trope we’ve used for years, in this situation, leads to an exponential function, which doesn’t look at all like setting up a proportion.

In either case, we get to make new connections about parts of math we’ve been learning about, and we get to see that (a) you can find functions that fit data and (b) often, there’s a good, underlying, understandable *reason* why that function is the one that works.

I will gradually enhance the pages on the eeps site to give more examples. And of course you can buy the book on Amazon! Just click the cover image above.

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## About Tim Erickson

Math-science ed freelancer and sometime math teacher. In 2014–15, at Mills College in Oakland, California.