The line y = k intersects the graph of y = 3x^{2} in points A and B. Points C and D are on the x-axis and ABCD is a rectangle. If the area of ABCD = 128/9, what is the value of k? Calculators not allowed. Show your method clearly!

The above question is designed for students in their second year of algebra or precalculus. It can also be used for practice for the Algebra 2 questions on the SAT, although it is somewhat above that level. Many students, including the more advanced, tend to struggle with problems like these because they don't have that much experience with coordinate geometry questions. These types of problems are critical for their later development. Once the students have done a few of these they do not find them so formidable. A useful pedagogical tool is to let them try it, review the method clearly, then erase the board and call on students to recall each step. Tell them you will do this, encouraging them to take good notes and pay careful attention! When using it on an assessment, make it a bonus the first time, then make it count. Some of my readers may recall a similar parabola problem a few months ago.

## Tuesday, July 31, 2007

### Another Quadratic Function Challenge

Posted by Dave Marain at 7:25 PM 15 comments

Labels: algebra 2, coordinate problems, quadratic function

## Monday, July 30, 2007

### Percent Word Problems Revisited -- Making It Easier?

Since there has been some discussion about the complications in the previous post on percents, how do we as educators deal with adversity and transform it into a teachable moment. We're explaining a difficult problem that students are struggling with, so we try to explain it again, but to no avail. What do experienced educators do?

(A) Abandon the problem - it was simply too hard or they're not ready for it yet. Perhaps assign it for extra credit?

(B) Make the question simpler by removing some of the complexity. Consider a scaffolding approach?

(C) Re-think what prerequisite skills were needed?

In this case, let's re-work the problem as follows:

In the senior class, there are 20% more girls than boys. If there are 180 girls, how many more girls than boys are there among the seniors?

What do you think the results will be now that we've concretized the problem?

Do you think some students will make the classic error of taking 20% of 180 and subtract to obtain 144 boys? You betcha!! There's no getting around the issue of recognizing the correct BASE for the 20% in my opinion. Whether you consider the boys to be 100% and the girls to be 120% or you let x = number of boys and 1.2x = number of girls, the central issue is recognizing that you cannot take 20% of the girls! Sometimes students need to just use algebra as a tool - it's a great one! By the way, one student used algebra with ratios to solve this:

Boys = x

Girls = 1.2x

Difference = 0.2x

Therefore, the difference/girls = 0.2x/1.2x = 1/6 and 180 divided by 6 equals 30!

How would you have reworded the question to make it more accessible?

Posted by Dave Marain at 10:21 AM 9 comments

Labels: percent, percent word problem

## Sunday, July 29, 2007

### Dad's Math Book Makes the Grade

Thanks to Joanne Jacobs for this interesting article about how an engineer, a native of Ghana and now living in Detroit, decided to give up his full-time job to dedicate himself to writing a series of math texts to help his own children (who were struggling) and now others. After reading the full article I must say I want to see more details and samples of what he's written. According to the article, his materials adhere to his state's standards and apparently blend the traditional algorithms and skill development with conceptual understanding. Gee, does this sound familiar! The fact that other parents and school administrators are now taking notice and want copies is fascinating. Textbook publishers pour tons of money into developing a new math series and along comes one individual who decided these materials were simply not working for his children. Is there a message here?

Posted by Dave Marain at 10:50 PM 0 comments

Labels: textbooks

### There are 20% more girls than boys in the senior class. What % of the seniors are girls? The confounding semantics of percents...

PLS READ THE COMMENTS. VERY ASTUTE OBSERVATIONS FROM OUR REGULAR CONTRIBUTORS TO WHICH I RESPONDED IN GREAT DETAIL. THIS MAY CONTINUE...

For now, I'll just leave the title as the problem to be discussed. Please consider how middle and high school students would approach this. How many might incorrectly guess that 60% of the seniors are girls and 40% are boys. If the problem were rephrased as a multiple-choice question, this type of error would occur frequently from my experience. What causes the confusion? Is it just the wording of the question or is there also an underlying issue regarding conceptual understanding of percents? Could it be that the question is asking for a percent and hasn't provided any actual numbers of students? What are the most effective instructional methods and strategies to help students overcome these issues? Certainly, algebraic methods would be a direct approach, but what foundation skills and concepts should middle schoolers develop even before setting up algebraic expressions?

Posted by Dave Marain at 6:49 AM 14 comments

Labels: algebra, middle school, percent, percent word problem, ratios

## Thursday, July 26, 2007

### Making Connections: An Algebra Problem -- Or Is it Geometry? Or Both?

Lines L and M are perpendicular. Line L contains (0,0). Lines L and M intersect at A(5,2). Find x.

Comments:

(1) Would students in geometry be more likely to consider some or all of the following: similar triangles, altitudes on hypotenuse theorems, areas, Pythagorean approaches? If this were presented as a coordinate problem in Algebra 2, what would be the most likely approach? Would some students use the distance formula?

(2) As always, it is our role as educators to present these kinds of challenges and to encourage students to think more deeply. Making connections between algebra and geometry happens naturally for some, but certainly not for all! We must enable this dialogue via the classroomm environment we establish and the kinds of questions we ask. It is important to first decide the goal. What concepts are we trying to develop? Slope? Ratios of corresponding parts? How many students never make the connection between the two!

(3) The answer is 29/5 or 5.8. Think of at least two methods!

Posted by Dave Marain at 11:53 AM 9 comments

Labels: algebra, connections, coordinate problems, geometry, similar triangles, slope

## Wednesday, July 25, 2007

### Understanding Algebra Conceptually?

What do you think would be the results of giving the following Algebra 1 problems to your students before, during, and after the course?

Do you believe that either or both of these could be or have been SAT questions?

Do students normally have exposure to these kinds of problems in their regular assignments?

Do these kinds of questions require a deeper conceptual understanding of algebra?

1. Given: x^{2} - 9 = 0

Which of the following must be true?

I. x = 3

II. x = -3

III. x^{2} = 9

(A) I only (B) I, II only (C) I, III only (D) I, II, III

(E) none of the preceding answers is correct

2. Given: (a - b) (a^{2} - b^{2}) = 0

Which of the following must be true?

I. a = b

II. a = -b

III. a^{2} = b^{2}

(A) I only (B) I, II only (C) I, III only (D) III only (E) I, II, III

Posted by Dave Marain at 11:44 AM 10 comments

Labels: algebra, concept, logic, SAT-type problems

## Tuesday, July 24, 2007

### Are all triangles in the universe similar to 3-4-5?

(a) AB = 8, BC = 6 in rectangle ABCD. Find the lengths of all segments shown in the diagram above.

Specifically: BD, CF,DE, BE, FE, CE

Comments: This is another in a series of rectangle investigations. To deepen student understanding of triangle relationships and to provide considerable practice with these ideas, the question asks for more than just one result. Students should be encouraged to first draw ALL of the triangles in the diagram separately and recognize why they are all similar! Using ratios, students should be able to find all the segments efficiently. One could also demonstrate the altitude on hypotenuse theorems as well!

(b) In case, students need a bit more of a challenge, have them derive expressions for all of the above segments given that AB = b and BC = a. To make life easier, assume b > a. This should keep your stronger students rather busy! This algebraic connection is powerful stuff. We want our students to appreciate that algebra is the language of generalization.

Posted by Dave Marain at 7:45 AM 5 comments

Labels: 3-4-5 triangles, altitude on hypotenuse, generalization, geometry, right triangles, similar triangles

## Monday, July 23, 2007

### How many even 3-digit positive integers do not contain the digits 2,4, or 6?

More practice for students...

As indicated many many times on this blog, there is no substitute for experience!

The keys to success here are:

(1) Careful reading (do students often miss the key word!)

(2) Knowledge of facts (why is zero the most important number in life!)

(3) Knowledge of strategies, methods (multiplication principle, organized lists, counting by groups, etc)

If these problems are helpful for students, let me know...

Posted by Dave Marain at 10:28 AM 4 comments

Labels: combinatorial math, multiplication principle, reasoning, SAT-type problems

## Wednesday, July 18, 2007

### Surviving the Math Wars - Both Ends Against the Middle

Identity Crisis...

I imagined the following dialogue taking place between a traditionalist (T) and a reformist (R) after both had finished reading the posts I've written on this blog for the past 6-7 months:

T: He is definitely a radical reformer. He uses phrases like investigations, explorations, discovery-learning, problem-solving, working in groups and encourages the use of the calculator for some activities. He is more concerned with conceptual understanding than with content and skills that all students must know. He actually believes that young children can think profoundly about concepts before they have completely mastered their skills. He talks 'less is more.' He has no documented research base for any of his wild ideas, and pretends that decades of classroom instruction are just as legitimate as a carefully developed research study. In what accredited journal of education research has he published?

R: Nonsense. He's one of your kind. He preaches strong foundation in basic skills, automaticity of basic facts, facility with percents, decimals and fractions and generally does not promote the use of the calculator in the lower grades. Other than some esoteric mortgage activity he wrote, most of his math challenges have little to do with the real world, focusing instead on number theory and combinatorial math, topics which are above the heads of most middle schoolers. His geometry problems are very traditional. He is often critical of calculator use. He focuses on standards and curriculum, even suggesting we need a nationalized math curriculum (and we know who will determine that!). He doesn't even applaud the efforts of his own national math teacher organization. He covers up his 'back to basics' approach by pretending he is a centrist. We all know one cannot be a centrist here. Either you're pregnant or you're not! He has no documented research base for any of his wild ideas, and pretends that decades of classroom instruction are just as legitimate as a carefully developed research study. In what accredited journal of education research has he published?

I know the regular readers of this blog know who I am and I know where their heart lies, but what can be done to move math education into a zone of reality that is sorely needed for our children. All these wonderful ideas but there is still so much confusion out there. How far have we come in the past dozen years or so to change the reality of a curriculum that is 'one inch deep and one mile wide'? Your thoughts are welcome...

Posted by Dave Marain at 7:28 AM 13 comments

Labels: balanced view, math wars

## Tuesday, July 17, 2007

### Developing Conceptual Reasoning in Geometry Via Standardized Test Questions

Ok, here's a fairly typical SAT-type of question that requires application of fundamental ideas in geometry. Some students 'see' a way to find the value of x in less than 15 seconds. These students have strong conceptual ability and are confident of their knowledge and reasoning. They are not thrown by a question that is somewhat different from the textbook problems they've seen.

Some of the issues as I see them are:

(1) How do we raise the knowledge/experiential base and confidence of those students who cannot seem to find a solution path and inevitably give up?

(2) How do we extend the thinking of those talented students who solve the problem in short order and just sit there complacently?

Perhaps, beyond these considerations and the instructional strategies employed is the bigger issue of PROVIDING FREQUENT CHALLENGES FOR OUR STUDENTS THAT GO BEYOND NORMAL TEXTBOOK FARE.

Where does one normally find these types of challenges in school curricula? Embedded in a natural way in the regular set of textbook exercises that can be routinely assigned? OR are they labeled as Standardized Test Practice at the end of a section or in a separate part of the chapter or text? OR are they found primarily in ancillary materials provided by the publisher? Can you guess where I think they should be and if they should be labeled? Should they be stand-alone multiple-choice questions or more open-ended with several parts that go beyond the 'answer?'

Before I suggest an activity based on this innocent-looking problem, I invite readers to consider a variety of methods of solution (so far I've observed about 6-7 'different' approaches) and how one might go beyond this standardized test question to deepen student reasoning. To remove the element of surprise and focus attention on process, the 'answer' is 90... (sorry!).

Have fun but if you 'see' the answer in 15 seconds or less, don't stop there!

Posted by Dave Marain at 7:44 AM 8 comments

Labels: geometry, reasoning, SAT-type problems

## Monday, July 16, 2007

### Oh No! - Not More Corny Math Jokes!

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I can't believe I'm sinking to these depths but it is summer and this may be the time for silliness. Two of my students shared the first couple (I'm doing it from memory so they're not exactly the originals) and the third I saw somewhere in my travels across the web (I forget the site to give proper attribution):

1. Who designed King Arthur's RoundTable?

Sir Cumference, of course. (ugh, groan,...)

2. What did pi say to i?

Get real!

3. Number two said to number three: "Boy, number 6 is really bad news. He's always in trouble."

Number three replied, "What do you expect. He's a product of our times!"

Ok, let me have it!

Posted by Dave Marain at 12:33 PM 4 comments

Labels: math humor

## Friday, July 13, 2007

### The 12th Carnival of Math on Friday the 13th!

Of course, there would be more harmony in the universe if this had been the 13th Carnival but enjoy the 12th Carnival nevertheless over at Vedic's Math Forum.

Vedic has selected an interesting assortment of posts covering a broad range of math. For example, Knot Homology in Algebraic Topology, Key Concepts in PerCents, Math Mnemonics, Observing Objects Near the Speed of Light, Sudoku and Graph Theory, An Interesting Way to Calculate the Square Root of 2, a discussion about finding Next Gen Math Teachers and a couple of recent posts from this blog. Enjoy! A great way to beat the summer doldrums!

The next Carnival (#13!) will be hosted on 7-27 over at Polymathematics.

Posted by Dave Marain at 5:52 AM 0 comments

Labels: carnival of mathematics

## Wednesday, July 11, 2007

### Two Coordinate Problems: SATs and Geometry Enrichment

Two of the opposite vertices of square PQRS have coordinates P(-1,-1) and R(4,2). (a) SAT-Type: Find the area of PQRS.

SAT Level of difficulty: 4-5 (i.e., moderately difficult to difficult).

Note: For standardized tests, in particular, students are encouraged to learn the special formula for the area of a square in terms of its diagonal.

Now for a more significant challenge that can be used to extend and enrich. Students can work individually or in small groups:

(b) Explain why there is only one possible square with the given pair of opposite vertices. Use theorems to justify your reasoning. Would this also be true if a pair of consecutive vertices were given? How many rectangles, in general, are determined if 2 vertices are given (opposite or consecutive)?

(c) Determine the coordinates of Q and S, the other pair of vertices of the square.

Note: There are many many approaches here. Students often get hung up on the distance formula leading to messy algebra (with 2 variables). There are simpler coordinate methods. You may want to provide a toolkit for students here: Graph paper, Geometer's Sketchpad, etc. Students who estimate or 'guess' the coordinates must verify (PROVE) that these vertices do in fact form a square. Students who quickly 'solve' the problem should be encouraged to find more than one method. This is the only way they will expand their thinking!

And now another coordinate problem that can be solved by a variety of methods...

In right triangle PQR, with right angle at Q, the coordinates of the vertices are:

P(-p,0)

Q(0,8)

R(r,0)

Determine the value of the area of the triangle. Assume p and r are positive.

Notes: Students should again be encouraged to try both synthetic (Euclidean) and analytical (algebraic, coordinates) methods.

General Comment: Students often forget how powerful slopes can be when solving geometry problems by coordinate methods!

Posted by Dave Marain at 4:59 PM 2 comments

Labels: coordinate problems, geometry, investigations, proof, SAT-type problems, squares

## Monday, July 9, 2007

### x = 3: It's a Point! No, It's a Line! No, It's a Plane! It's... It's SuperPlot!

Sorry for the silly title but when the temperature approaches 100, I start becoming delusional!

Algebra teachers know that the equations of horizontal and vertical lines (the coordinate axes in particular) are stumbling blocks for students and creative educators and desperate students often resort to clever mnemonics and other memory aids to recall these. I invite readers to share their favorite. The teachers in my department (former department that is -- be kind, I'm adjusting to retirement) became enamored of HOY-VUX. I'm not sure who originated it but this person deserves credit! The name is silly (reminiscent of horcruxes from Harry Potter), the students laugh at it, but when the test comes around, they write it at the top of their paper. Here's how it works:

HOY: Horizontal, slope 0, Y=...)

VUX: Vertical, Undefined slope, X=...)

Now I know that others out there have their favorite ways of teaching this so PLEASE SHARE!

Believe it or not, the above was not the intent of this post but it's probably more interesting than the technical stuff to follow! This discussion is intended for Algebra 2 students and beyond. A full treatment requires some vector analysis but I will avoid that for now. Unlike most of my offerings, I did not set this up as a worksheet but you'll get the idea. You may want to bookmark this and save it for when 3-dimensional graphing comes up in the curriculum.

Start with a horizontal number line: <------------------|---------------->x

Ask students to plot x = 3 on the line. No ambiguity here, right!?!

Thus, the 1-dimensional graph of x = 3 is a POINT! Easy, so far.

Now draw both coordinate axes. Plot the point at 3 on the x-axis, ask a student for both of its coordinates and ask if it satisfies the equation 1x + 0y = 3.

Confirm this: 1(3) + 0(0) = 3.

Students generally treat x = 3 as an exceptional case of the equation of a line, but having both variables may help them see it isn't that special (other than its slope of course!).

Ask students to verify that (3,1), (3,2), (3,-1), (3,-2) all satisfy the equation 1x + 0y = 3.

Have them plot these points.

Ask the class (I didn't feel like writing this in the form of a worksheet today) to verify that (3,k) satisfies this equation for any real value of k. Students need to understand this significance of the zero coefficient of y.

This should help them to recognize that the graph of x = 3 is a vertical line. Don't get me wrong. Understanding this does not necessarily lead to getting it right on a test! They still need survival gear (mnemonics) for that! HOY-VUX to the rescue!

BUT THERE'S MUCH MORE TO THIS!

Point out that in the equation 1x + 0y = 3, we see that the resulting line is PERPENDICULAR to the axis with the non-zero coefficient (x-axis here) and PARALLEL to the axis whose coefficient is zero (the y-axis in this case). We're not proving anything here or explaining why this is true, just making an observation that we will generalize later.

Ok, so where's the plane in all of this?

In 3-dimensional space, we examine the equation 1x + 0y + 0z = 3. We can still graph the point corresponding to 3 on the x-axis, the vertical line graphed above (y can be chosen arbitrarily) and now z can be any real number. Corresponding to each variable whose coefficient is zero, the graph will now be a plane PARALLEL to that axis and PERPENDICULAR to the axis whose coefficient is not zero. Thus, our graph is now a plane parallel to the y- and z-axes (therefore parallel to the yz-plane determined by these axes) and perpendicular to the x-axis. Of course, software like Mathematica, Derive, or even freeware available on the web will help students visualize this better. Cardboard or Styrofoam models are also highly effective here. The more the students construct these models and label the axes and planes, the better they will be able to make sense of all this.

So what is the graph of x = 3? All of the above!

Now have your students analyze the equation y = 3 following this model!

Posted by Dave Marain at 3:37 PM 4 comments

Labels: 3-dimensional graphing, algebra 2, investigations, linear equation, math mnemonics

## Friday, July 6, 2007

### 7-7-07: Your Lucky Day?

Of course, July 7th, 2077 or 7-7-77 will be fun too for those around to enjoy it 70 years from now!

Now you all know the story of the inveterate gambler who waited until the 7th day of the 7th month to bet $777 on horse #7 in the 7th race at 7-1 odds. Of course, the horse finished 7th! Sorry, I couldn't resist telling this groaner...

Posted by Dave Marain at 7:23 PM 11 comments

Labels: math humor

### If the difference of 2 numbers is less than the sum of the 2 numbers... Developing logical reasoning in our students

If the difference of 2 numbers is less than the sum of the 2 numbers, which of the following must be true?

(A) Exactly one of the numbers is positive

(B) At least one of the numbers is positive

(C) Both numbers are positive

(D) At least one of the numbers is negative

The answer given in the original source was (B). Do you agree? See notes below for further discussion of the wording of the question (before you react!).

This SAT-type question was posted about a year ago in my discussion group, MathShare (which is still extant but possibly being phased out). On that forum, I discussed how students struggled with the subtleties of logic in their analyses of the problem. That online discussion led to a meaningful debate (involving some exceptionally thoughtful educators) about how and when logical thinking needs to be developed in our students. All agreed that critical thinking and logical reasoning must begin when children enter school, long before the formalism of an axiomatic approach. Do you believe this is currently happening in most elementary schools? What materials are being used by those districts or teachers who are infusing critical thinking and logic? If we move toward a more standardized curriculum nationally, how important is this? I'm sure you know how I feel!

Other Notes about the problem above:

(1) Is the question ambiguous or flawed because the term difference fails to specify in what order the numbers are subtracted? Should the domain of numbers be specified (would integers be better?). On the SAT, it is understood that the domain is always real numbers.

(2) Why do you think so many students (these were strong SAT prep students) struggled with this and had great difficulty accepting that (B) was the correct choice? Do you think phrases like at least one and exactly one are problematic for many students?

(3) What methods do you think were used by students? There were several approaches as I recall.

(4) Do you think students should be encouraged to use an algebraic approach here rather than plugging in numbers and testing various cases?

(5) Would restating the question in its contrapositive form make it easier to grasp? (how many students remember this from geometry?!?)

(6) Would this question lead to a richer discussion if it were open-ended, i.e., no choices given?

(7) Could this question be given to middle-schoolers after they have learned the rules of integers or do you believe they do not have sufficient maturity to handle the logic?

Your thoughts....

Posted by Dave Marain at 6:41 AM 10 comments

Labels: critical thinking, logic, SAT-type problems

## Monday, July 2, 2007

### The Multiplication Principle - Developing SAT/Math Contest Skills

How many even 4-digit positive integers greater than 6000 are multiples of 5?

Students who have had many experiences with problems like this have a huge advantage on Math Contests and SATs. To level the playing field, you might want to consider giving your middle and secondary students a daily SAT/Math Contest Problem of the Day like the one above. Questions like this require:

(a) Careful reading skills (encourage underlining or circling of keywords

(b) Knowledge of the Fundamental Principle of Counting (most often termed the Multiplication Principle)

(c) Clear thinking

(d) Careful reasoning

The answer is 399 (pls correct this if you feel I erred!). The process is equally important. Students should be encouraged to list a few examples (preferably the first 2-3) of numbers satisfying the conditions. The challenge is to recognize HOW MANY conditions are subtly embedded in the dozen or so words in the question ! Some students will prefer to make an organized list and count by grouping, which is fine, but, as they develop, they should recognize that is the basis for the Multiplication Principle (and later on, permutations).

Posted by Dave Marain at 7:32 AM 17 comments

Labels: counting problems, middle school, multiplication principle, SAT-type problems