Polynomial Discovery!

I love teaching polynomials because I think they offer an opportunity for students to see the rule of 4 constantly (graphical, numerical, algebraic, and verbal). I’m including several links and documents that I use in developing the ideas during the unit.

End-Behavior Lab:

In Precalculus we start our polynomials unit with a short review of the rules of end-behavior in a lab format. I like this because it gets into an expscreen-shot-2017-02-08-at-3-17-41-pmloration of the “WHY” at the end.

end-behavior-lab-short-version

The first part is just sketching what the students notice when they change the leading coefficients for polynomials of different degrees. There are links to sketches of polynomials of different degrees like this one:screen-shot-2017-02-08-at-3-21-06-pm

 

Repeated Roots Lab

Screen Shot 2017-02-08 at 3.15.36 PM.pngHere we talk about multiplicity and how repeated roots affect the behavior of the function. I use the language that the function “bounces” off the axes at zeros with even multiplicity, and it “lags” through zeros with an odd multiplicity greater than 1.

repeated-roots-lab

In addition to students noticing the behavior of these functions, I ask them to see if they can determine the degree, the leading coefficient, and constant term without expanding the entire polynomial. They use wolframalpha.com to check their answers to that question.

Why the “flat butt” and “sharp arms”?

The next class, I have the students do a little more exploring graphically and numerically. The question is: When a factor is repeated more times, WHY does the graph get “flatter” on the bottom and “sharper” sides?

They explore using desmos, and they look at graphs like this:

screen-shot-2017-02-08-at-2-23-13-pm

Why is the “butt” flat and why are the “sides” sharp?

 

I also ask them to make a table of values screen-shot-2017-02-08-at-2-57-50-pmfor the final function, and do the calculations by hand (or at least x=2.8 and x=3.2).

I want them to remember that: repeated multiplication by a number whose magnitude is >1 will give a number whose magnitude is larger (larger than the original), whereas repeated multiplication by a number whose magnitude is <1 will be a number whose magnitude is smaller (smaller than the original).

So the major take-away is that: in the polynomial once I subtract 4, the difference will be repeatedly multiplied by itself, and if it’s taken to the 16th power it will behave much more dramatically than if it’s taken to the 2nd power. 1.2^16 is much larger than 1.2^2, and 0.8^16 is much smaller than 0.8^2.  The “flat region” corresponds to the x-values within 1 unit of the zero, and the “sharp arms” correspond to x-values beyond 1 unit of the zero.

Time to Problem Solve:

I am not big into teaching processes if I don’t have to. I’d rather have my students discover or wrestle with the math, so I have the following problem as a warm up on a particular day. The students are using what they know to write an equation for a particular polynomial:

polynomial-thinking-problem

The trick with this one is that most student can get the correct factors for the polynomial, but they need to do a little work to find the leading coefficient.

Creating Polynomials:

In this activity, I have the students reinforce what they know about polynomials so far, and they get to use Wolfram Alpha to expand polynomials in factored form to polynomials in standard form.

creating-polynomials-acitivity

 

What else do I do?

I have a lot of white board space, so we spend time practicing finding zeros, expanding polynomials, and drawing sketches of polynomials.We also spend time considering how many zeros and what type–those are fun problems because they require the students to explain verbally their reasoning.

screen-shot-2017-02-08-at-3-29-30-pm

I love this because students have to explain their reasoning.

screen-shot-2017-02-08-at-3-29-49-pm

I also require them to determine how many rational vs. irrational zeros there are. In this case, both zeros are irrational!! Even though you might sware that the polynomial crosses at 2. However, since 2 is not a factor of 9, 2 cannot be a zero (according to the RZT).

MOSTLY, I care about the students making the connection between the algebraic and the graphical representations.