# Ep 69: Equations of Lines are Figure-out-able!

October 12, 2021 Pam Harris Episode 69
Math is Figure-Out-Able with Pam Harris
Ep 69: Equations of Lines are Figure-out-able!

We've talked a lot about the types of reasoning we need to help students develop, but a lot of teachers believe that there comes a point in higher math, where reasoning just won't cut it. In this episode Pam and Kim tackle functional reasoning, and demonstrate that algebra can be so much more than memorized steps.
Talking Points:

• The importance of working with intuition
• Finding the equation of a line without formulas
• Intuitively finding the equation of a line in standard form.
Kim Montague:

Hey fellow mathematicians. Welcome to the podcast where math is figure-out-able. I'm Pam. And I'm Kim.

Pam Harris:

And we make the case that mathematizing is not about mimicking steps or memorizing facts. It's about thinking and reasoning; about creating and using mental relationships. We take the strong stance that not only are algorithms not particularly helpful in teaching, but that mimicking algorithms actually keep students for being the mathematicians they can be. We answer the question, if not algorithms and step by step procedures, then what?

Kim Montague:

Alright, y'all In this episode, this week, we are going to do some higher math. Just for you all.

Pam Harris:

Whoo! All right. And Kim's a little excited. I mean, scared. I mean excited about today. We were kind of talking a little bit about how this episode was gonna go. And Kim was like, here we go.

Kim Montague:

This is not my jam. But I'm willing.

Pam Harris:

Yet!

Kim Montague:

But I'm going to learn and grow.

Pam Harris:

Kim Montague:

Yeah.

Pam Harris:

Can we work on that today?

Kim Montague:

Sure. Do it.

Pam Harris:

I wondered in that moment, could we teach that? Could we teach that strategy of looking at the values, finding a pattern, and then just recording that pattern sort of, generally. Okay, so I'm going to give you - alright listeners, Kim, I'm going to give you guys a set of points, I'm going to give you all a set of points. And then I'm going to pause and I'm going to suggest that you pause and then think to yourself, what are the patterns that you see in that set of points? Do you see any patterns? Specifically think about if I gave you any x value, could you do something to that x value and tell me what the y would be so that it would follow the same pattern as the four points that I'm going to give you?

Kim Montague:

Okay.

Pam Harris:

You might even graph the points just to like sort of prove to yourself that they're on the same line. However, caution. If you're a high school math teacher, you might not want to do that. Because if you graph those points, you might not be able to stay out of a rote memorized way of writing the equation of a line. So if you've got some rote memorized way of writing the equation of a line, I'm gonna encourage you to maybe not do that, to see if there's some patterns that you can find. And maybe don't graph them. If you already have a - go ahead if you want to. But maybe if you don't want the surprise spoiled, maybe don't, maybe don't do that. But anybody who doesn't have that down, you don't have a rote memorized procedure down to write the equation of a line, maybe just like what patterns do you see. Maybe even graph the points and see kind of what they look like? Do they lie on the same line? And what does that mean to you? And then come back. Alright, Kip, here's the first set of points. Four points. So the first point is zero, negative two. So I would write that as (0,-2), right? Okay, so an ordered pair zero, negative two. The second point is one, negative one. The third point is two, zero. And the last point is three, one.

Kim Montague:

Okay.

Pam Harris:

So y'all pause. Letme say it again. (0,-2), (1,-1), (2,0), (3,1). Pause the recording, go see what patterns you can find. Maybe graph the points. Come on back. All right, hopefully people paused a little bit. Okay, so Kim, what patterns... What patterns do you see just like first thing that comes to you? What patterns do you see?

Kim Montague:

I noticed that the x values increase by one.

Pam Harris:

Sure enough.

Kim Montague:

And the y values also increased by one.

Pam Harris:

Sure enough, like they're going up. Now they're different, right? They're not the same. But they're all going up by one. Okay, cool. Did you notice any other patterns?

Kim Montague:

Yes. So I, I also looked, I don't know if you say it this way, but kind of between the points. Between the X and Y value. And I noticed that -

Pam Harris:

From x to y?

Kim Montague:

Yeah. So from x to y is a decrease of two.

Pam Harris:

What do you mean?

Kim Montague:

So I'm looking at the series of points that you just gave us and I started kind of from the bottom up, and I said, how do I get from three to one, and that's subtract two. How do we get from two to zero? That's subtract two. How do we get one to negative one? That's subtract two and to get from zero to negative two, that's subtract two.

Pam Harris:

So I hear you saying that (3,1) was actually kind of helpful as you looked between the x and y. You said yourself, Well, I know that relationship, that's just minus two. I wonder if it works for the other problems? I kind of heard wonder.

Kim Montague:

Absolutely.

Pam Harris:

And woo! It did. You're like, yeah, it did. So I might record your thinking, I might make that visible by saying you took an x value, subtracted two. So I would write down x - 2. And you said every time that equaled y.

Kim Montague:

Yes.

Pam Harris:

So I would write down x - 2 = y. And you just wrote the equation of the line line that contains those points. I don't know if you could hear the drumroll there. Now, just from noticing a pattern, you were able to like, Hey, I found a pattern that works for all the points. And so I'm just going to generalize that. I could give you any x value. Like what if I gave you four Kim? Now if I gave you four, what would you do?

Kim Montague:

I would say that the X is four and the Y is two.

Pam Harris:

Because?

Kim Montague:

Because four minus two is two.

Pam Harris:

Yeah, so maybe that's not the best point because we got too many twos in there. So what if I give you five? What if I say the x value is five? What would the point be?

Kim Montague:

The point would be (5,3)

Pam Harris:

Because you're saying x subtract two is always going to give you the y value. There you go. So I could literally give you 1,000,000. 1 million is the x value. And you could say that the y value would be

Kim Montague:

198,000.

Pam Harris:

Oh

Kim Montague:

Oh, I did that. I started to write it down. 999,998.

Pam Harris:

There we go. That was fun.

Kim Montague:

Hilarious.

Pam Harris:

That's what I would typically do. Okay, cool. So you've now found a pattern that works for any x value. And that is what the equation of a line means. It means this is a pattern that if I give you any x value, you can tell me the y value based on this pattern. That's how we describe that set of ordered pairs, that set of points that, that make up that line.

Kim Montague:

Okay

Pam Harris:

So ready for the next one?

Kim Montague:

Sure.

Pam Harris:

All right, here we go. four points again.

Kim Montague:

Okay.

Pam Harris:

And you're gonna look for patterns and try to tell me if I give you any x, what would the y be, Okay, first point, negative one, one. So negative one comma one. Next point, (-1,1), (0,0). (1, -1), (2,-2), Let's say it really fast. (-1,1), (0,0). (1, -1), (2,-2) All right. Pause. Pause the recording. Pause the podcast. Go see what patterns you can find. Come back when you're ready. All right. Hopefully they paused the recording. I feel funny, because I feel like I should pause longer.

Kim Montague:

Yeah, because I don't have a huge pause.

Pam Harris:

Oh, that doesn't give you any pause. Kim's like, Pam! Give me some time here!

Kim Montague:

Well, okay, so can I tell you the first thing that I thought about? Because I was trying to think what this would look like on a graph. And so I, I like the origin, right?

Pam Harris:

That's nice. Yeah, Zero zero, is in there.

Kim Montague:

But so when I was looking at the first and the third points that you gave me, I was like, Oh, those are opposites. So -

Pam Harris:

Those? So those are adjectives. What's opposites?

Kim Montague:

Sorry, the negative one, one and the one negative one.

Pam Harris:

I just said adjective and meant pronouns. Oh, so the x value is opposite of the y values?

Kim Montague:

Right. Mhmm.

Pam Harris:

Is that true for all the points?

Kim Montague:

Mhmm.

Pam Harris:

Okay, so I might record your thinking that the y value is, equals, the opposite, negative, of the x value. I might write that as y = -x, or the opposite of x. Because you're telling me that the y values always opposite the x value, so if I give you any x value, take the opposite sign, I would get the y value.

Kim Montague:

Yeah.

Pam Harris:

And that's the equation of that line. Y = -x. Or the opposite of x. No way, right?

Kim Montague:

Yeah.

Pam Harris:

And let me just pull back on some prior knowledge that you might have. I think you know what the line y = x looks like, right? It's that 45 degree line. If I were to graph it, that's the line y =x. You and I've done some work that I think you know that parent function. So what if I just said, Kim, that y equals x, I need all the opposites now, of the y values. Every time you had a positive y value, when you went over to an x value, every time you had a positive y there, I need now - don't graph the positive y. At that x value graph the negative y value. I wish you could see my hand because I was like putting a point up in the first quadrant. And I was like, but not that positive y value. And I was like reflecting down into the fourth quarter to stick it down there. But back in the, let's go in the normal y = x. That's normally in the third quadrant, right? It's down there, that 45 degree angle down there, negative negative third quadrant. But now those y values shouldn't be negative because I need the opposite of them. I need them to pop up to the second quadrent. I need them to be positive. Now y'all if you haven't heard quadrants for a while and everything, okay, sorry. But hopefully to all of my graphing people that graph more often that made a little bit of sense that we can sort of talk through why that reflection is happening. All right, let's do another one. Here we go. I'm gonna give you four points. And (-2,3), (-1,2), (0,1), (1,0).

Kim Montague:

Okay, they're gonna pause.

Pam Harris:

Oh, yeah, sorry. I was pausing. Pause the recording. All right. So Kim's gonna pause. Oh, so do you want us to really pause longer here and Craig can cut out the -

Kim Montague:

No, it's okay.

Pam Harris:

Craig's our editor everybody. He cuts dumb things out, we're not gonna have him cut this out. We're just gonna let Kim think.

Kim Montague:

It's live math.

Pam Harris:

Kim's like, stop talking.

Kim Montague:

Okay, okay. Okay. All right. So I, you know, I find that now I'm not really necessarily looking from the first point down to the second point down to the third down the fourth, like I did the first time. Because I'm recognizing that you want me to think about how to get from x to y. So that original thing I did has not helped me. So I want to first look from zero to one and I want to call it plus one. But then that's not really helpful for - so x + 1 - but that's not really helpful for the other points.

Pam Harris:

Okay.

Kim Montague:

I'm looking at -1 to 2. And how can we get from negative one to two? And I'm thinking that the only way to make something work for both of those points is to consider the opposite of x.

Pam Harris:

That's actually interesting. When you said negative one to two, I thought, well, you could add three.

Kim Montague:

Yeah, I did.

Pam Harris:

But I'm assuming you did that. And discounted it -

Kim Montague:

I did.

Pam Harris:

- because it didn't work for any other points. Yeah. So I just wanted to, like, bring that out. So so then you said for me to get to get from a negative value to a positive value... That maybe there might be some of the opposite stuff going on. And we could have mentioned that in the problem before, when we had y equals the opposite of x that we did, we had this sort of switchy thing happening between the x's and the y's, we had kind of this negative/positive switch happening. And so there was the opposite of x in the problem before. So in this problem, you also might have some opposite of x going on.

Kim Montague:

Yeah. And so you know, it's interesting, because I feel like the the zeros are actually tripping me up a little bit. If I would have focused on the first two, then, then it's definitely the opposite of x plus one. Let me confirm. Opposite of x plus one.

Pam Harris:

You just tried it on the first two problems.

Kim Montague:

Yep, sure. did.

Pam Harris:

Yeah. So like when you look at (-2.3) - di I just interrupt you?

Kim Montague:

Pam Harris:

When you looked at negative two, three you said, well, if I looked at the opposite of negative two, that's two. Well, to get from two to three, that's just plus one. Yep. Let's see if it works on the next. Opposite of negative one is one. One plus one is also... bam! And then you can go check to see if it works on the zero. And often zeros and ones are funky.

Kim Montague:

Okay,

Pam Harris:

Often zeros and ones are a little - they behave a little differently because of the identity properties. And so that's interesting. Yeah, those can absolutely trip you.

Kim Montague:

Yeah, I was drawn to the zero at first, but now I'm-.

Pam Harris:

Sure. So I would record your thinking as, I can get any y value - y equals - the opposite of x, so I would write -x and then add one, plus one. So the equation of the line could be y = -x + 1. Totally cool. If I may share a different strategy for the same problem.

Kim Montague:

Okay.

Pam Harris:

I wish I could - this is our unnamed gal, our science gal at the workshop. This is the strategy she used. So she looked actually at the zero and the one, even though you found them problematic, she was like, Okay, if I look at the point (0,1) and I look at the point (1,0), both times, if I add those together, I get one. Like zero plus one is one.

Kim Montague:

Okay,

Pam Harris: