Teaching Exponents In Eighth Grade, Part 1: [Old To The Now] - The Jose Vilson

Teaching Exponents In Eighth Grade, Part 1: [Old To The Now]

September 11, 2012

India Power Outage

Every so often, I’m inserting some posts on pedagogy, especially for those of us who aren’t as math-inclined. If you find this helpful, just let me know in the comments.

Today, I’m differentiating between the old way I used to prepare the students for a lesson on exponents and the way I do it now.

Old Way

1. Ask “What’s an exponent?”
2. Here are the five mail rules you need to know.
3. Apply them.

New Way

1. Ask “What’s an exponent?”
2. Use their definitions with a bit of refinement.
3. Get their thinking about a few cases involving exponential, expanded, and standard forms.
4. Have them deduce what 3¹ and 3° power would look like.
5. If they can see the operation of division, then prompt them to ask if this works for every case in general.
6. Use the same thinking from 3-4 to get the values for 3¹ and 3°.
7. Let them establish the (real) first two laws of exponents.
8. Use similar tabular thinking to deduce product, quotient, and power of a power laws.

Usually, when students are just given the laws, there’s rarely reason why we give it to them. It gets shoved on their laps, especially x to the zero power. Some of it might be because of a lack of understanding on the part of the teacher, but I also suspect it’s because we don’t see the importance of taking them through similar steps that an actual mathematician would go through.

But I changed that last year, and into this year.

My main objective was to make sure to establish the foundation for every argument we will make about exponents from here until October.

Here’s a sample of what I mean:

Exponential Expanded Standard
x3 x · x · x n/a
x2 x · x n/a
x1 x n/a
x0 1 1

 

I’m hoping for a couple of things here. First, I hope they see that, as the exponent decreases, the amount of x’s in the expanded column decreases, just to solidify our definition of “exponent.” Also, I hope they see some sort of division (making the connection between multiplication and division matters here). I use a specific case first, and it’s usually a positive base, just so they’re familiar with the numbers, then work my way towards this.

By the way, once they see the 1 at the end, they should also be able to see that Icould have had a multiplication of 1 all along since 1 times any number equals that number.

Hope that helps. Best,

Mr. Vilson, who will have a post like this about once a week …

This post was written by...

José Luis Vilson is a math educator, blogger, speaker, and activist. For more of my writing, buy my book This Is Not A Test: A New Narrative on Race, Class, and Education, on sale now.

For more about me, read here.

{ 2 comments… read them below or add one }

Anne V September 11, 2012 at 10:23 pm

Of course, it works if you extend it into negative exponents as well! I was an engineering major before I switched into education. I remember explaining zero exponents to the rest of my elementary math education class. No one had ever bothered to show them!

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Jose September 11, 2012 at 10:26 pm

Exactly right, Anne. You took the words out of my mouth (and the post I was going to write in a few weeks). I’m hoping that they can see the relationship between the negative exponent and the fractional ways I write it. Let’s see how craftily I explain it. -rubs chin-

Thank you.

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