SL2RL-Math247

8 March 2009

Baddie of the Month – Teaching complex numbers with the quadratic formula

Filed under: 8-12,algebra,applets,education,K-12,math — Tags: , , , — admin @ 12:14 am

Goodie: “A technique/question that can be applied in many places and teaches thinking.”

Baddie: “A technique/question that is a waste of good teaching and learning-to-think-and-do-math time.”


March 2009 Baddie of the MonthTeaching complex numbers with the quadratic formula.

Who was the dingbat who first decided to work complex numbers when teaching the quadratic formula?

And why? Because you can get complex conjugate numbers from the quadratic formula?

A car has brakes. Do we teach hydraulics to someone learning to drive a car?

Storyline: We are teaching quadratics. Everything we discuss is totally real. (In fact, we usually "fix" our problems to be with integers, but that is subject of previous rants.)

We are factoring, finding intercepts, drawing graphs in the Cartesian plane – all real numbers.

In the middle of this, we start teaching a totally different subject – namely complex numbers.

And then we go back to real numbers and real applications of quadratic functions.

Let’s face facts.

(a) Complex numbers have NO relation to quadratic functions or their applications that we will work on.

(b) Complex numbers have NO visual representation on the graph of a quadratic function*.

Ergo – complex numbers do NOT help us understand quadratics.

Conclusion: Do NOT mention complex numbers at all when teaching the quadratic formula.

Simply state that when D<0 (negative discriminant) the quadratic function has no roots and therefore does not cross the x-axis. Don’t mention "real roots". Don’t go anywhere else with this discussion at all.

STICK TO THE SUBJECT MATTER AT HAND – Quadratic functions and their applications.

—————–

This is the material in a typical textbook in the chapters for quadratic functions. (This is actually a reasonable toc. Some left me gasping for breath.)

Let’s go. We only have 2 months. No problem – we will fix everything to be integers.

  1. Here is a quadratic function y= x². Let’s make a table of points. (Hmm – I’ve only ever graphed a line. I know I can graph a line using any 2 points. What the heck are all these points?)
  2. The graph of a quadratic function is a parabola. (Hmm – So a parabola is the graph of a quadratic function?)
  3. Here is the general expression of a quadratic y=ax²+bx+c, where x and y are variables and a, b and c are constants. (Hmm – They all look like letters to me.) Also, m²-2km+k² is a quadratic. (Huh?)
  4. Let’s factor ax²+bx+c. Here are a bunch of rules. (Dang, I can’t factor this quadratic x²-3x-2. Teacher said it was because I copied wrong x²-3x+2. Do these rules work?)
    Here are some more rules. (Hmm – Why do we want to factor? What are those numbers anyway?)
  5. Let’s complete the square. Here is the plan. The point (h,k) is the vertex. (Hmm – Why is it the vertex? Why is there a minus in front of h and a plus in front of k?)
  6. Let’s graph quadratics by completing the square and transforming the graph of y=x². (Say what? You go left when? First upside down? And then stretch?)
  7. Here is the quadratic formula. We prove it using completing the square. (Wow, look at all those letters and equations. Now – square root. Plus and minus sign. Never seen that operation before – cool.)
  8. Using quadratic formula, let’s find the roots of a quadratic. (Roots? Is there a function here? Is there an equation here? Linear functions have roots?) The roots are the factors. (Hmm. Roots look easy to find. Couldn’t we just factor that way and skip all that factoring stuff?)
  9. Let’s graph a quadratic by completing the square. Now use the quadratic formula to find the roots. The roots are the x-intercepts of the function. (Hmm. Roots look easy to find. Aren’t parabolas symmetric? Why can’t I just find the vertex by going halfway between the roots and skip all that completing the square and transforming the function?)

Okay – I am sure they got all that. Let’s pause and do a totally different subject.

  1. D is called the discriminant. D can be positive, zero or negative. If D is negative, the quadratic function doesn’t have real roots. (Real roots? Are there fake roots?)
  2. If D is negative, the quadratic function has complex roots, which are complex numbers. You remember: z=x+iy. Complex roots come in pairs called complex conjugates.
    *Wait – we can make this worse. Let’s graph complex numbers and their conjugates in the plane. (No kidding – my son did this in the middle of learning to graph quadratics.)
    (Whoa – I thought we were talking about quadratic functions and graphing parabolas. What do I get with complex conjugates? Where do I put these on the graph? Whaaaaaaaaaat?)

Now back to quadratics. Back to the reals – are we totally confused yet?

  1. Now let’s look at applications of quadratic functions.

Related topics:

Past: February 2009 Goodie of the month – Good questions for Quadratic Equations/Functions

Past: February 2009 Baddie of the month – Teaching completing the square to graph a quadratic.

Past: January 2009 Baddie of the Month – Hand-factoring a quadratic with a≠1.

22 February 2009

Goodie of the Month – Fun and Learning with Quadratics

Filed under: applets,education,ICT,math — Tags: , , , , , — admin @ 3:23 am

Goodie: “A technique/question that can be applied in many places and teaches thinking.”

Baddie: “A technique/question that is a waste of good teaching and learning-to-think-and-do-math time.”


February 2009 Goodie of the MonthReal Fun and Learning with Quadratics

In the second half of Algebra 1:

A typical standard is: Apply quadratic equations to physical problems, such as the motion of an object under the force of gravity.

A typical question for this is: A ball is thrown straight down with a speed of 20 [ft/s] from a height of 80 [ft]. When will it hit the ground?

A typical application of technology is:

Tell the student that the function is y(t)=-16t2-20t+80. They know they can’t graph with t so they switch to x, which they graph on their graphing calculator.

They see that parabola crosses the x-axis. They find the intersection and write x=1.7 and get their points.

Now ask them "Where does the ball hit the ground?". They will point to the intersection point – totally forgetting that this is vertical motion and that the ball hits the ground at (0,0)!

Ask them "What is the units on your answer?". You will be lucky if they give you [seconds] and not [feet]!


So why is this a “Goodie of the Month”?

The problem isn’t the standard. Nor is it the question. Both are excellent. The problem is the technology – it is undoing the learning.

Let’s change the technology!  If the animation below doesn’t work – open this link: Vertical Motion


 

 

 

 

 

 

 

 

Sorry, the GeoGebra Applet could not be started. Please make sure that Java 1.4.2 (or later) is installed and active in your browser (Click here to install Java now)

Here are some “good problems”.

  • Set ho = 80[ft] and vo = -20[ft/s]. Run the animation. Point with your finger to the place where the ball hit the ground. Now find the place on the graph where it says when it hit the ground.
  • Set ho = 80[ft] and vo= 20[ft/s]. Run the animation. Notice that the ball goes up before it goes down. Why is this? Reset the animation and using the step forward + and step backward – buttons, stop the animation when the ball is at its highest point. Point with your finger to the place where the ball is at its highest point. Now find the place on the graph where it says when it is at its peak. What time is this?
  • Set ho = 0[ft] and vo= 100[ft/s]. Find when the ball hits the ground. Do this via the animation and algebraically using the function. When is the ball at its highest point (remember – parabolas are symmetric!)? What is this highest point? Do not forget units!

Here are some “good questions” for the function: h(t)=ho+vot-16t2

  • The function h(t) gives height in [ft]. So each member of this function must give [ft].

    • ho is (initial) height. So its unit is [ft]. It is all by itself so this member is in [ft].
    • vo is (initial) velocity. So its unit is [ft/s]. How does this member give [ft]?
    • What do you think the unit of “16” is so that this last member gives [ft]?
  • In what part of the function is gravity playing a part? In which of the above problems is the only force gravity?
  • Why do you think there is a plus sign in front of vo and a minus sign in front of 16? That is, what does it mean in mathematics/physics for an object to have a positive velocity? Does gravity increase this velocity?
  • Make up a problem that describes this situation: ho = 0[ft] and vo= 100[ft/s].

Links for this interActivity (worksheets, downloads, etc.): Open Metadata

8 February 2009

Baddie of the Month – Teaching “completing the square” for quadratics

Filed under: education,math — Tags: , , , , — admin @ 4:13 am

Goodie: “A technique/question that can be applied in many places and teaches thinking.”

Baddie: “A technique/question that is a waste of good teaching and learning-to-think-and-do-math time.”


February 2009 Baddie of the MonthTeaching “completing the square” for quadratics

Yes, she is stuck on quadratics. But quadratics are so useful and can be fun and we keep teaching techniques that don’t give much value for the time and effort.

Why? These are reasons I have found online or been given "for" studying “completing the square”:

(1) one can then prove the quadratic formula , (2) one can find the vertex of the quadratic,
(3) one can graph quadratics using graph transforms and (4) one can solve integrals of the form dx/(x^2+bx+c)

My response:

  1. Students at this level cannot understand a mathematical proof. All they see is a manipulation of symbols/letters/numbers. Ask any non-math student. Ask any non-math adult. They didn’t get it. Period. So we teach them a technique in order for them to see a proof they don’t understand. (I am happy if they get accurate results from the quadratic formula with any numbers for a, b and c and can relate them to the graph of the quadratic and/or the answers to the question that was posed and judge the reasonableness of all.)
  2. It is much easier and useful to find the vertex of a quadratic by first realizing that every parabola is symmmetric and thus the vertex must be the value of the function at the half-way point between the roots*. Finding the vertex this way requires them to relate solutions from the quadratic formula to roots/zeros/x-intercepts, reinforces learning about midpoints, relates the vertex to the quadratic formula – more reinforcement  and it teaches them to find function values.  Win-win-win-win. Using "completing the square" teaches them to manipulate numbers.
  3. Does anyone actually graph a function using a graph transform – ever? After trying to teach graph transforms for over 20 years – I have decided that the best I can hope for is "a vague understanding" that "x^2+4" is "x^2 up 4" and 3sinx is "3 times taller than sinx" and cos2x is "2 times faster than cosx". Trying to combine all these is a total waste of time.
  4. Solve integrals – good grief. Maybe we should teach them partial fractions in algebra 1 too.

How to find the vertex: The x-value of the vertex is x= -b/2a. Substitute this value into the quadratic to get the y-value.

(BTW: nowhere did I find the semi-plausible defense that “completing the square” is also used to find the center of a circle and identify conic sections in the 2nd half of Algebra2. By then, I agree that completing the square is not an unreasonable technique to teach.

*roots found using the quadratic formula I am sure :) .


Related topics:

Past: January 2009 Baddie of the Month: Hand-factoring a quadratic with a≠1.

Future: Baddie – Teaching complex numbers in the same 2 month span as you teach graphing of quadratics.

Future: Goodie – Good questions for Quadratic Equations/Functions

25 January 2009

Goodie of the Month – A Good Question for Algebra 1

Filed under: algebra,applets,education,ICT,math — Tags: , , , , — admin @ 2:01 am

Goodie: “A technique/question that can be applied in many places and teaches thinking.”

Baddie: “A technique/question that is a waste of good teaching and learning-to-think-and-do-math time.”


January 2009 Goodie of the MonthA Good Question for Algebra 1

Two ships are sailing in the fog and are being monitored by tracing equipment. As they come into the observer’s rectangular radar screen, one ship, the Rusty Tube, is at a point 900 mm to the right of the bottom left corner of the radar screen along the lower edge. The other ship, the Bucket of Bolts, is located at a point 100 mm above the lower left corner of that screen. One minute later, both ships’ positions have changed. The Rusty Tube has moved to a position on the screen 3 mm left and 2 mm above its previous position on the radar screen. Meanwhile, the Bucket of Bolts has moved to a position 4 mm right and 1 mm above its previous location on that screen.
Assume that both ships continue to move at a constant speed on their respective linear courses. Using graphs and equations, find out if the two ship will collide.

Why do I like this question?

  • Students can understand it and it is fun.
  • They can graph it on paper or using a graphing program.
  • It involves finding the equation of a line through 2 points (twice) – good reinforcement.

Why do I think it is a “good question”?

  1. The graph looks like every 2×2 system of linear equations they have solved in Algebra 1.
    • It looks like the boats collide at the intersection point (see below).
    • It seems like all they need to do is solve the system and be done.
    • … until you say “Where is time on the graph?”.
  2. The student can build a animated simulator that “shows time” – easily!
  3. The kids can make the boats collide – what fun!.
    • They can move the starting points until they get the boats to collide.
    • They can also adapt the simulator so that they can change the slopes and get the boats to collide. Directions here.
    • My thanks to David Cox for seeing this!
  4. You can get all kinds of mathematics out of them.
    • You can get them to calculate when each of the boats reaches the intersection point in the original question.
    • You can get them to check the math on their “colliding simulator” to see if the boats really do collide, where and when.
    • You can ask them about a 3D graph and what this would look like when the boats don’t collide and when they do.

To animate, click on the play button at bottom left of graph.

To animate manually, right-click on slider and deselect “Animation on”. Then, click and drag the point on the slider.

Sorry, the GeoGebra Applet could not be started. Please make sure that Java 1.4.2 (or later) is installed and active in your browser (Click here to install Java now)

Source: I found this question asked on answers.yahoo.com. My webpage for this question is: mathcasts.org/mtwiki/Gq/BoatCollide

10 January 2009

Baddie of the Month – Factoring a Quadratic with a≠1

Filed under: applets,education,math,sloodle — Tags: , , , , , — admin @ 5:13 am

Goodie: “A technique/question that can be applied in many places and teaches thinking.”

Baddie: “A technique/question that is a waste of good teaching and learning-to-think-and-do-math time.”

I am going to try to blog a baddie and a goodie per month. We shall see and of course – this is my opinion.


January 2009 Baddie of the MonthFactoring a quadratic with a≠1 “by hand”.

Okay, I can mostly understand learning to factor “by-hand”:   x²+3x+2  or   x²+x-2.  

Once you understand the principles and get the technique (my scheme), factoring a quadratic by hand with a=1 is faster than using the quadratic formula.

But, I absolutely and totally do not understand the reasoning behind other factoring-by-hand techniques!

Why not: Factoring techniques

  • serve no useful purpose – once factored, with a≠1 you must still solve the individual factors.
  • don’t always work – MOST quadratics even with a=1 and real roots CANNOT be factored by hand.
  • are hard to learn, there are many “special cases”, they take alot of time to teach, …

What to do and why: Use the quadratic formula for all your factoring needs.

  • We are going to teach them the quadratic formula anyway.
  • It always works – either we get real roots and can factor or we get non-real roots and know we cannot factor.
  • By using the quadratic formula all of the time, the connection between quadratics, roots, x-intercepts, graphs of quadratics becomes clear.
  • Repetition of a single technique is much more likely to stay in their heads.

Conclusion: Don’t teach factoring by hand except when a=1. Use the quadratic formula.

Here’s how: ax²+bx+c=a(x-x1)(x-x2) where x1=(-b+D))/2ax2=(-b-D))/2a,  D=√(b²-4ac)

(Here “by hand” means looking for the factors without a formula like when you say “The factors of 2 are 1 and 2 and oh yes, they add to 3 (first expression) or the factors of 2 are 1 and 2 and oh yes, they subtract to 1 (second expression)”.)


Related topics to come in future blogs

Please, OMG please don’t teach completing the square – an even worse waste of time than hand factoring.

Please, please don’t teach complex numbers in the same 2 month span as you teach graphing of quadratics.

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