Powers, Roots, and Logs are Related Facts

This year, in my Algebra 2 class, I prefaced our individual function units with a overview of functions in general. One thing that happened is that before we studied exponential functions, we had a decent understanding of inverses and how several functions are related in this way. Several weeks before I ever needed to hint at the existence of logarithms, the students saw the need for an inverse to an exponential function and also were stymied by the relationships that are already comfortable to them: namely, the existing inverse relationships between powers and roots.

So this month, when we got to the middle of our exponential function unit, I decided to present logarithms as a group of THREE related facts in a fact family.

We listened to a fabulous Radiolab program that presents numbers and logarithmic thinking as a human interest story. I am so grateful for programs like this that do my hard work for me!

Then we talked about how powers, roots, and logarithms are all different ways to say equivalent things, while each highlighting a different feature.

Fact families are something that students are familiar with. No one batted an eye.
When I asked them what is meant by logarithmic thinking, I was happy with how their explanations centered around exponents and thinking about 'how many times a number is doubled or tripled,' etc.


We concluded with some fact practice by using fact triangles and naming the three related facts. I made these awesome octahedral dice with fact triangles in base 2, base 3, base 4, and base 5. Their job? Roll a die and record the three facts that can be written from the trio of values.

The students whipped through it, never complained, and had 100% accuracy on our quiz the following day. I'll count it as a worthy addition to my filing cabinet.

Algebra 2 is All About Exponents

I don't know if this makes function families easier or more complicated, but I realized this week that everything we cover in Secondary Algebra (1 and 2) can be reduced to two basic function families: f(x) = n^x and f(x) = x^n (trig functions excluded, but we don't cover those until precalc at my school). I don't remember this ever getting pointed out to me when I studied functions, and seriously, how many years have I been teaching this? I just find it very interesting.

We made these organizers in class today (and yesterday). We've already got our brains wrapped around transformations and compositions, although we have thus far stealthily avoided talking about operations on functions (adding/ subtracting and multiplying/dividing)... other than to say, "Ugh, two x's, that looks messy." Which of course, it IS, right? And isn't that the point?  At least, I think that's my point this time through. Basic functions that involve these families, simple transformations, and compositions where one step follows another in a specified order... this kind of function is not hard to work with. They are logical and orderly. It's only when we start multiplying and dividing or adding/subtracting functions that stuff gets tricky and we need to start pulling out new tricks like factoring and zero product rule, and complex numbers, and extraneous solutions, and reducing rational expressions, and limits, oh my.

Why aren't exponents and roots always inverses?

We've been studying functions in my Algebra 2 class. I'm taking an entirely new approach this year. It's going well, but the jury's still out. So far, one thing I'm really happy about is their excellent grasp of inverse functions as a process of 'undoing' whatever the original function did. Today in class we were organizing our thoughts about the different function families. We'd drawn out some nice examples of expoential growth and decay and I asked if we could figure out the inverse functions. I EXPECTED them to see that they could not write a function (we haven't done logs yet), but that they could use the tables and graphs that they just made to draw an inverse graph. Our function was a simple "2 to the power of x."

But they surprised me. "The inverse would be the xth root of 2," someone said.

Several agreed. I was dumbfounded. Why not?

After all, isn't it TRUE that:

for all real values of n?

That is, I know there are some issues with this as a blanket statement. For example, even values of n only work if we restrict the function domain. But for the most part, this is entirely true and logical.

So why is it FALSE that:

Maybe you, like me in class today, are scratching your head now and wondering... could that be right? It isn't. We checked. Choosing an input for the original function, applying the function and then applying the inverse function will not return us to our original value. But WHY?

Help me out here. Can anyone provide a purely sensible argument for why this will not work? Not just a demonstration of HOW it doesn't work. I can supply several of these now. I want to know why.

Number Line Movement as a Function Intro... Continued

I've been using a new approach to introduce functions to my Algebra 2 students (who presumably aren't completely new to functions). As much as I can, I'm trying to let need dictate the math. At this point our definition is somewhat incomplete, but working well so far. Today we see a need for more explicit notation, because it's not always clear to us what exactly the function rule is:

I was amazed how quickly they got comfortable with function notation, given this set-up, but I am willing to accept that it may be partly due to previous teachers and previous foundations.

From here we jumped right into the idea of function compositions (but not composition notation, because we don't need it yet):

It was entirely logical to follow these with a discussion of inverses that are a bit more complex:

Here are two samples of their student work at this point. There still seems to be some ambiguity around the word opposite (a great entry point for an upcoming lesson). Beyond that, however, their understanding appears to be rock solid. Everybody did a nice job of explaining the function. Only one got mixed up on defining the inverse. A teacher's dream.

Number Line Movement as a Function Intro

I'm bragging here, and these ideas are not all mine, but I did an introductory lesson on functions for an Algebra 2 class today and it rocked. My students are average juniors in high school and we've just finished our first unit on Sequences and Series. We're about to dive into an intense study of polynomial, exponential, logarithmic, rational, and radical functions. I wanted to set the stage with a solid foundation of how functions behave - including transformations, compositions, and inverses. Here's what today looked like.

The students walked into class and there was a large number line taped to the floor of the classroom. I asked for volunteers.

discovering additive identity and translations through number line movement

Now they know what to do. So they go back 'home' and then they subtract 3, rest, and then add three. Ah. The inverse is born:

introducing function inverses through number line movement

What comes next turns out to be mind blowing, and even I didn't expect it. We return 'home' and then multiply by 2:

It seemed so odd to everyone that a simple operation like 'multiply by 2' would result in such seemingly UNuniform motion... which made it all the more amazing to 'discover' the constant rate of change in the distances between neighbors.

Now seemed like a good place to introduce some traditional algebra:

We followed these ideas with a wonderful exploration of our tables and graphs generated from our movement activities. Thank you Desmos, for a wonderful tool. We were able to easily see both our data points and our generalized lines, and the symmetry of inverses was so obvious and yet so cool nevertheless. You can check out our exploration here: https://www.desmos.com/calculator/ecfpw2h4tp

The last item of the day was to define some function terminology:

In the end, we had a good start to to a definition of function:

A function is a relationship between two variable quantities that follows a rule to map inputs to outputs.

It needs work, but they don't know that yet. Tomorrow we'll get into function notation and simple compositions and inverses that use only the traditional operations as they were presented today. Then, we'll return to the number line and explore some other function rules: opposites, reciprocals, absolute value, exponents, roots, etc. When we run into trouble (I PLAN on it!) we'll adjust accordingly.


(Thank you to Max Ray and Michael Pershan for the inspiration from their Teaching Complex Numbers workshop at NCTM 2015. I'll be going back to these ideas when I get to complex numbers, for sure.)