## Tuesday, October 31, 2017

### Lesson 5-3: Conjectures (Day 53)

This is what Theoni Pappas writes on page 304 of her Magic of Mathematics:

"The 8 rows and 8 columns of black and red squares composing a checkerboard can be grouped into different sized squares. These squares range in size from 8 * 8 to 1 * 1. How many squares of various sizes can be found on a checkerboard?"

This is the third page of the checkerboard subsection. Pappas now gives her puzzle on "The Squares of a Checkerboard."

You don't need to see the picture, which is just a checkerboard with some of the squares of various sizes drawn in. As usual, I'll post the answer tomorrow.

Meanwhile, here's the answer to yesterday's question. We wish to divide a checkerboard into 2 * 2 squares using folds and a single cut. As it turns out, we must fold it diagonally to solve this one.

Step 1 is to fold each of the four vertices in towards the center of the board. The result is a square of 32 -- that is, each of its sides has length 4sqrt(2).

Steps 2 through 5 is to fold this resulting square in half the usual way -- that is, forming rectangles at each step. The final figure is a square of area 2. We then cut this square along its diagonal -- which you may notice is of length 2. Therefore this single cut produces 2 * 2 squares after unfolding.

This is a good puzzle to give during the current Chapter 5 of the U of Chicago text, since it's all about quadrilaterals, including squares and rectangles.

Lesson 5-3 of the U of Chicago text is called "Conjectures." There is no real equivalent in the modern Third Edition, as Chapter 6 of that text goes directly from the equivalent of the old Lesson 5-2 to the equivalent of the old Lesson 5-4.

This is what I wrote two years ago about today's lesson:

Lesson 5-3 of the U of Chicago text discusses conjectures. The text defines a conjecture to be "an educated guess or opinion." It is a statement that has yet to be proved.

David Joyce has a low opinion of conjectures in a high school geometry class. In his scathing review of the Prentice-Hall text, Joyce writes:

(And this occurs in the section in which 'conjecture' is discussed. "Test your conjecture by graphing several equations of lines where the values of m are the same." What's the proper conclusion? That theorems may be justified by looking at a few examples?)
In summary, the material in chapter 2 should be postponed until after elementary geometry is developed.

That is to say, the statements should be postponed until they can be proved.

Now Michael Serra takes a diametrically opposite approach from Joyce. In Serra's Discovering Geometry text, a great many statements are given well before they are proved -- since the proofs don't occur until the final three chapters of the book, Chapters 14-16 (old version -- Chapter 13 in the new version). What would Joyce say about having so many of these unproved theorems in his text?

The theorems can be proven once a little actual geometry is presented, but that's not done until the last half of the book. A little honesty is needed here. Why not tell them that the proofs will be postponed until a later chapter? Or that we just don't have time to do the proofs for this chapter. Even better: don't label statements as theorems (like many other unproved statements in the chapter).

Well, Serra doesn't label statements as theorems -- he calls them conjectures! In all, 114 conjectures are in Chapters 3-13 of Serra's text -- Conjecture 1 is the Perpendicular Bisector Conjecture (which we've already proved on this blog) and Conjecture 114 is the Law of Cosines. Clearly, discovering and stating conjectures are the heart of Serra's learning philosophy.

As I've stated before, I want to lean towards Joyce's preference of proving all theorems as soon as they are mentioned. I've rearranged the order of the U of Chicago text in order to make sure that proofs precede applications of the theorems -- especially when I want to highlight the Common Core proofs, which may differ from traditional proofs.

But I'm also sympathetic to Serra's philosophy. I want to show some interesting results of geometry without being limited to what the students can prove. Also, one way to pique a student's interest is to show the result and ask, "Is this always true?" or "Why is this always true?" Proving many boring low-level theorems just to make sure that nothing is used before it is proved would result in students losing interest and wondering why they are forced to write endless proofs. And of course, Serra's text helps out students who may be weak at proof-writing and would easily forget how to write a proof once taught -- the main results are stated so the students can learn them, and then the proofs are given at the end of the book, right before the PARCC exams.

The point of this lesson is to get the students thinking about the properties of special quadrilaterals without worrying about how to prove them. In other words, I want to get the students engaged and thinking about the quadrilateral properties so that they can make the conjectures.

We begin by dividing the class into groups -- say of three or four students. Each group is assigned a worksheet -- or the members can write down answers on a common blank sheet. Then my usual set of ten questions are assigned -- but there are some differences between this and the usual individual worksheets that I post.

First of all, let's look at the first two questions:

1. What is the teacher's __________?

2. What is the teacher's __________?

Beforehand, the teacher fills in the blanks with words -- I'd fill them in with age and weight. I have no problem with giving this much information to the students -- but many people, especially women, are highly sensitive to revealing such personal data. This is why I left blanks in the questions -- so that the teachers fill in the blanks with words that they are comfortable revealing in class.

The teacher asks the question, "What is my age?" (or whatever is in the first blank). The groups signal when they want to answer. The teacher calls upon the group that signaled first to answer -- and since this answer will almost certainly be wrong, the teacher then calls upon another group. When a group finally gives the correct answer, the teacher awards this group a point. (In case you're as curious as the students are about my age, I am currently 36 years old.)

Notice several things about this game so far. The first team to give a correct answer -- and the answers in my version of this activity are numerical so far -- is the one to get the point. And after the first two questions, two groups have one point each -- or possibly one team already has two points -- and the rest have none.

Certainly the groups without points so far are eager to earn one. And so they are faced with the next question in the activity:

3. True or false: the diagonals of a rectangle are always equal in length.

Recall that this activity is all about conjectures. The students have already spent time making conjectures (that is, educated guesses) about the teacher's age and weight -- now it's time to make a conjecture about geometry!

This question serves several purposes. First, the students in groups that are trailing in points -- the same students who would have complained about doing math after the long exam -- now suddenly want to answer a math question because they want to catch up to the leaders. Second, this question is a true-or-false question, so students who might have tuned out if given an open-ended question will want to try this one at least since there are only two possible answers. The students are likely to guess at the answer -- and they're encouraged to do so, because a conjecture is a guess! Third, the conjecture in question involves rectangles -- and students who tend to forget what a rhombus or trapezoid is will still remember what a rectangle is. The only problem word that might be a barrier to participation is diagonal -- so the teacher reminds them that the two diagonals of a rectangle run from a corner to the opposite corner.

In my activity, every third question (that is, the third, sixth, and ninth) is a true-or-false question. I use these to give the students more opportunities to earn points. The teacher allows every group to give an answer of true or false before revealing the answer, and every group that gives the correct answer earns a point. In this way, groups can earn points without worrying about being the fastest group to get the answer.

Of course, the answer to Question 3 here is true. Hopefully, most, if not all, of the groups were able to guess that the diagonals of a rectangle are equal, so that every group is on the scoreboard. Now we move on to the next questions.

4. The diagonals of a square always divide the square into four triangles of __________ size.

5. The diagonals of a kite are always __________.

Now these questions are open-ended, just like the first two questions (but there are no more personal questions -- from now on, all are geometric). So we return to having the groups compete, and only one group will receive the point.

The students should test the fourth conjecture by drawing several squares -- by several, let's say one for each member of the group. So the first group to have drawn enough squares for the group as well as give the correct answer "equal" is the group to earn the point. Since these problems are increasing in difficulty, a teacher may choose to give two points, rather than one. (Notice that the four triangles are in fact congruent, but since congruence has not been taught yet, we instead say that they have equal "size" -- where the students can probably get an idea of what that might mean.)

The difficulty in the fifth question is that after having seen the diagonals in Questions 3 and 4 turn out to be equal, the students may jump to the conclusion that "equal" is correct yet again. The teacher should remind the students to draw the kites to make sure -- and to drive the point home, the teacher should draw a counterexample to the claim that the diagonals of a kite are equal. The students might not think to say that the diagonals are "perpendicular," which is the correct answer. So the teacher can give the hint that they should check the angle between the diagonals. By now, I'd award the point(s) to the group telling me that the angle is 90, even if the actual word perpendicular is not used.

Now we move on to our next true-or-false question:

6. True or false: consecutive angles in a parallelogram are always equal.

Of course, the teacher should define consecutive angles (or "adjacent angles"). If necessary, the teacher can draw a parallelogram on the board and point out where the consecutive angles are -- and naturally, that parallelogram should be a rectangle (or nearly so), in order to avoid giving away that the correct answer is false.

7. If ABCD is a parallelogram and angle A has measure 30, then angle B has measure _____.

8. Opposite angles in a parallelogram are always __________.

Notice that the seventh question is an extension of the sixth -- ABCD is a parallelogram, and the measure of angle A is 150 degrees. Because of question 6, the answer isn't 150 degrees. A point (or points) are awarded to the group correctly answers that the angle B measures 30 degrees. No conjecture is stated in this question, but the implied conjecture is that the consecutive angles in a parallelogram are supplementary. Since the answer to question 7 is numerical, I don't require the groups to draw a parallelogram for each student in the group.

Of course, the answer to question 8 is that opposite angles in a parallelogram are always equal.

9. True or false: opposite sides in a parallelogram are always equal.

This one is self-explanatory -- the answer is also true.

10. A square has _____ lines of symmetry.

I've mentioned this one earlier on this blog. A square has four lines of symmetry -- if the sides of the square are parallel to the coordinate axes, then a square has one horizontal, one vertical, and two diagonal lines of symmetry. The diagonals are the ones that are often missed. Once again, this is a numerical answer, so I don't require a diagram for each member of the group. Of course, the students will want to draw at least one square in order to find its symmetry lines.

And now, as I often like to do, here's a Bonus Question. As I pointed out last week, I don't like it when students are eliminated from passing when there is plenty of time left in the semester, and in the same manner, I don't want students to be eliminated from winning this game too early. And so this question can be worth many points -- enough for the last (or maybe the next-to-last) place team to catch up. (In my game, I may deduct points for behavior -- so I might not want the last place team to be able to win if their behavior doesn't warrant it.)

Bonus Question: Take a quadrilateral and find the midpoints of its four sides. Join these four points to form a new quadrilateral, the Midpoint Quadrilateral. Midpoint Quadrilaterals are always what type of quadrilateral in the hierarchy?

Students will need time to figure out how to draw this Midpoint Quadrilateral. After each member of the group draws the Midpoint Quadrilateral, the winning team will be the one that correctly identifies the Midpoint Quadrilateral to be a "parallelogram."

[2017 update: Notice that Step 1 of the folding from yesterday's Pappas puzzle is a Midpoint Quadrilateral. And meanwhile, today is Halloween. It's a good day for a semi-activity day, even though there will be another full activity day coming up on Friday. For the rest of this post, I write about two memorable times that I played this game in class. One was as a sub, in fact a few days after Halloween. The other was as an actual teacher, last year on my birthday.]

[First, as a sub....]

But yesterday, I had an opportunity to play my game. Most of the eighth grade math classes were assigned to take notes, but the Math/Computing class had a worksheet to finish. And so I began by asking the students to guess my age, and then my weight. So two of groups already had a point, while the other seven were scoreless.

Then the third question I asked was simply the first question from the worksheet -- namely to graph the equation x + y = 5 using intercepts. Just as I mentioned from my original Conjectures worksheet, every third question was a chance for each and every group to earn a point. I think that only about half of the groups earned the point. Some of the groups drew the graph incorrectly, while others had the correct graph but identified the slope as 1 instead of -1. My fourth question was the second question from the worksheet, x + 2y = 8 -- which, just as planned, allowed for only one group to earn the point.

I admit that graphing isn't necessarily the best sort of question for this game -- especially when the students had to do so much work to answer each question (finding both intercepts and the slope). The game worked out better on Monday, when the Computing class was working on the computers and the rest of the classes had equations to solve, so I played the game only in the other classes. When solving equations, nearly all the groups earned the point on the third question, which is what I want.

The worksheet consisted of about a dozen graphs, yet I only had time to play the game with six. So someone might point out that if I had simply passed out the worksheet and asked the students to work on it, they might have completed many more than six of the graphs. The game wastes so much time when the students can't work on the next problem until the class reaches it -- especially on every third question where I must check every group before proceeding.

But let's recall the situation the class was in. It was the second straight day the class had a sub. So the students, already knowing coming in that there was a sub, were already thinking about how much mischief they could get in -- things they'd never dream of doing with the regular teacher. And once they arrived, they were probably hoping that they could play around on the computer, only to find out that the teacher had locked the computers away and assigned the worksheet.

So we can see that the students weren't in much mood to work. And yet, I believe that there was a game, the allure of earning points motivates them. Many of the students might have just thrown the paper away, or worked on it at a snail's pace and still be on the first graph late into the period. Also, checking every third question keeps those who might have drawn every graph incorrectly or calculated every slope with the wrong sign.

Some people -- especially traditionalists -- dislike group work, and believe that students learn much, much more effectively when doing individual work. But I often find that as a sub, classroom management is easier when I only have to keep track of nine groups, not 35 students whose names I don't know. In order to earn a point, every member of the group must have drawn the graph on the paper, and so the students end up motivating each other at least to draw the graph on the paper (even if all they're doing is copying the other group members).

I remember that the group who debated with me about a slope of 1 vs. -1 failed to earn a point for that third question, and consequently fell behind the other groups. Even though by the end of the fourth question, I'd convinced them (using Slope Dude) that their slopes had the wrong sign, the group never caught up to the leaders. As I announced the winners, I overheard a member of that group saying to one of his teammates, "But we're the real winners because we did eleven problems and the others did only six." Yes, imagine that! The group kept on working on the graphs well ahead of my pace in an effort to catch up to the other groups' score. (This was my intent of including the Bonus Question on my original worksheet.) And these are students who would -- had I not played a game -- at best, have calculated all the slopes with the wrong sign, and at worst, have just thrown the papers away and not worked on the graphs at all!

And so I will continue to play this game in class. One difference between my original vision and the way the game has played out in the classroom is that I almost never make the questions for the entire class to answer true-or-false. After all, how often will the students be given a worksheet where every third question is true-or-false? One thing I might consider is, on every third question, have the groups work on a problem, and then present one group's answers to the class. Then all of the groups must determine whether that answer is correct (true) or incorrect (false). Indeed, even if I don't explicitly ask a true-or-false question, I might said to the group that insisted that the slope was 1, "Well, this other group says that the answer is -1. So let me take a point away from them and give it to you." So this threat would force the other group to defend their answer of -1 -- now the groups are debating the answer with each other, rather than the usual situation of me vs. the students.

Notice that I never actually give this game a name. I'm prone to name this game either Conjectures -- after the post in which it appears -- or Who Am I? -- which is the first part of the title of the post that appeared the day after I first posted the game. The only problem with either name is that they both refer only to the first two questions -- unless it's from my Conjectures worksheet -- since, for example, the fact that the slope is -1 isn't merely a "conjecture," but should be known. Of course, even on the Conjectures worksheet, "Who Am I?" only refers to the first two questions.

By the way, what exactly was the prize that I gave the winning groups? Actually, all I did was leave their names for the regular teacher with a positive note!

[Second, as a teacher....]

Now I decide to play this game today in all my classes. And you may ask, why today? Well, I actually played this game as a sub one year ago today -- and I did it for one very particular reason.

The answer to the first question "What is the teacher's age?" is 36. That's because today is -- you guessed it (or remembered from last year) -- my 36th birthday! And so I knew that if I was going to play a game which starts with my age, it might as well be on my birthday.

What lessons do I include in today's game? Well, just as in the version of the game I posted as a sub, I want to focus on geometry questions. As it turns out, the game fits the current seventh grade lesson like a glove. Yesterday, the students cut out triangles out of straw, and Illinois State even asks the students to make conjectures about the triangles they created. So it's easy to fit some of those right into the game.

Today is Wednesday -- always a scheduling adventure at our school. For once, we actually follow the same schedule as last week -- but again, it means that I don't see the seventh graders as much as the other grades. I try having them come up with Triangle Inequality as a conjecture. A few of them are able to get on the right track, especially after I give them the hint (or "lead them by the nose," much to David Joyce's dismay).

For eighth grade, I notice that the STEM project mentions the measures of angles that are vertical, adjacent, corresponding, and so on. So I play the game using these conjectures. One big problem is that some students can't use a protractor correctly, so many don't arrive at the conjecture that vertical angles have the same measure. (Actually, the seventh graders also had to conjecture Triangle Sum, but I don't even try to reach that conjecture, knowing that if the eighth graders won't use the protractor correctly, neither will the seventh graders.)

Meanwhile, for sixth grade, the animals project ultimately relates to guessing how much room animals need, so it fits into the game as well. They are learning about how to find the dimensions of a rectangle given its area -- that is, factoring.

I like this game as a sub because it gives the students something to do. But if I use it in the regular classroom, it might be better to do some preparation. Once again, I just took the STEM project and added my own "What is the teacher's age?" questions. But instead, I could have come up with some questions such as just measuring random given angles. If I award points in the game, then the students should be motivated to find them. Then after that I segue to finding specific angles such as vertical angles or those of a triangle. That should lead them to make the conjectures.

Returning to 2017, recall that this was on my birthday, not Halloween. On the October holiday last year, I tried to do another seventh grade Illinois State project, "Orienteering." I made the activity fit the holiday by dressing up as a pirate and having the students search for my treasure. I wrote on the blog that the activity didn't work as well as I wanted, because the public school (co-located with our charter) restricted us to a small area of the playground. It might have worked better this year now that we're no longer co-located -- if only I were still a teacher there.

## Monday, October 30, 2017

### Lesson 5-2: Types of Quadrilaterals (Day 52)

This is what Theoni Pappas writes on page 303 of her Magic of Mathematics:

"Using your visualization skills and folding techniques, determine a way to make one straight cut to separate the checkerboard into 2 * 2 squares, such as this one."

This is the second page of the subsection "Checkerboard Mania." Once again, the first page of this section was blocked by the weekend.

But we don't need to see the first page -- nor, for that matter, the pictures on this page -- in order to understand the problem. The checkerboard is a standard 8 * 8, and we are being asked to fold it so that a single cut will divide it into sixteen 2 * 2 squares. Pappas describes it as "taking a checkerboard apart with one fell swoop."

As usual, I'll post the solution tomorrow. This problem isn't easy at all -- the folds and the cut to make is extremely clever.

Lesson 5-2 of the U of Chicago text is called "Types of Quadrilaterals." In the modern Third Edition of the text, quadrilaterals appear in Lesson 6-4.

This is what I wrote two years ago about today's lesson:

Lesson 5-2 of the U of Chicago text covers the various types of quadrilaterals. There are no theorems in this section, but just definitions. The concept of definition is important to the study of geometry, and in no lesson so far are definitions more prominent than in this lesson.

The lesson begins by defining parallelogramrhombusrectangle, and square. There's nothing wrong with any of those definitions. But then we reach a controversial definition -- that of trapezoid:

Definition:
A quadrilateral is a trapezoid if and only if it has at least one pair of parallel sides.
(emphasis mine)

Just as with the definition of parallel back in Lesson 1-7, we have two extra words that distinguish this from a traditional definition of trapezoid -- "at least." In other textbooks, no parallelogram is a trapezoid, but in the U of Chicago text, every parallelogram is a trapezoid!

To understand what's going on here, let's go back to the first geometer who defined some of the terms in the quadrilateral hierarchy -- of course, I'm talking about Euclid:

http://aleph0.clarku.edu/~djoyce/java/elements/bookI/defI22.html

Of quadrilateral figures, a square is that which is both equilateral and right-angled; an oblong that which is right-angled but not equilateral; a rhombus that which is equilateral but not right-angled; and a rhomboid that which has its opposite sides and angles equal to one another but is neither equilateral nor right-angled. And let quadrilaterals other than these be called trapezia.

Of course, the modern term for "oblong" is rectangle, and a "rhomboid" is now a parallelogram. The word "trapezia" is actually plural of "trapezium." In British English, a "trapezium" is what we Americans would call a trapezoid, but to Euclid, any quadrilateral that is not a parallelogram (or below on the quadrilateral hierarchy) is a "trapezium." But the important part here is that to Euclid, a square, for example, is neither a rectangle (oblong) nor a rhombus. He makes sure to say that a rectangle (oblong) is "not equilateral," and that a rhombus is "not right-angled." And of course, neither a rectangle nor a rhombus is a parallelogram (rhomboid).

These are called "exclusive" definitions. For Euclid, there was no quadrilateral hierarchy -- each class of quadrilaterals was disjoint from the others. But since the days of Euclid, more and more geometry texts have slowly added more "inclusive" definitions.

One of the first inclusive definitions I've seen was the definition of rectangle. It was mentioned on an episode of Square One TV, when a Pacman parody character named Mathman was supposed to eat rectangles, and then ate a square because "every square's a rectangle." I would provide a YouTube link, but I haven't found the link in years and it doesn't come up in a search. (I even remember someone posting in the comments that just as for me, his first encounter of the inclusive definition of rectangle was through watching that clip when it first aired so many years ago!)

But many of my family members were also teachers, and one relative gave me an old textbook that still mentioned some exclusive definitions. In particular, it declared that a square isn't a rhombus. A little later, my fifth grade teacher then taught the inclusive definition of rhombus. I then blurted out that a square isn't a rhombus, then actually brought the old text to school to prove it! She replied, "Wow!" but then, if I recalled correctly, told me that this definition was old, and that by the new definition, a square is a rhombus. And so all modern texts classify the square as both a rectangle and a rhombus, and that all of these are considered parallelograms.

So we see that there is a tendency for definitions to grow more inclusive as time goes on. (We see this happening in politics as well -- for example, the definition of marriage. But I digress.) And so we see the next natural step is for the parallelogram to be considered a trapezoid.

One of the first advocates I saw for an inclusive definition of trapezoid is the famous Princeton mathematician John H. Conway. He is best known for inventing the mathematical Game of Life, which has its own website:

http://www.conwaylife.com/

But Conway also specializes in other fields of mathematics, such as geometry and group theory (which is, in some ways, the study of symmetry). Twelve years ago, he posted the following information about why he prefers inclusive definitions:

http://mathforum.org/kb/message.jspa?messageID=1081135

The preference for exclusive definitions arises, I think, from
what I call "the descriptive use". Of course, one wouldn't DESCRIBE
a square table as "rectangular", since that would wantonly use
a longer term to convey less information. So in descriptive uses,
there's a natural presumption that a table called "rectangular"
won't in fact be square - in other words, a natural presumption
that the terms will be used exclusively.

But the descriptive use is unimportant to geometry, where the
really important thing is the truth of theorems. This means that we
should use a term "A" to include "B" if all the identities that
hold for all "A"s will also hold for all "B"s (in the way that
the trapezoid area theorem holds for all parallelograms, for
instance).

You might worry about the consistency of switching to the
inclusive use while other people continue with the exclusive one.
But there can be no consistency with people who are inconsistent!
I've seen many geometry books that MAKE the exclusive definitions,
but none that manage to USE them consistently for more than a few
theorems.

Indeed, Conway advocated taking it one step forward and actually abolishing the trapezoid and having only the isosceles trapezoid in the hierarchy! After all, there's not much one can say about a trapezoid that's not isosceles -- just look ahead to Lesson 5-5. There's only one theorem listed there about general trapezoids -- the Trapezoid Angle Theorem, and that's really just the Same-Side Interior Angle Consequence Theorem that can be proved without reference to trapezoids at all. All the other theorems in the lesson refer to isosceles trapezoids. In particular, the symmetry theorems in the lesson refer to isosceles trapezoids. (Recall that Conway's specializes in group theory -- which as I wrote above is the study of symmetry.) I suspect that the only reason that we have general trapezoids is that they are the simplest quadrilateral for which an area formula can be given.

This is now another digression from Common Core Geometry, so I'll just provide another link. Notice that here, Conway also proposes a hexagon hierarchy based on symmetry. There's also a pentagon hierarchy, but there are only three types of pentagons -- general, line-symmetric, and regular -- just as there are for triangles. It's easier to make figures with an even number of sides symmetric.

http://mathforum.org/kb/message.jspa?messageID=1074038

Another advocate of inclusive definitions is Mr. Chase, a Maryland high school math teacher. (And no, in yesterday's post and today's I'm referring to two differentMaryland teachers.) I see that he is so passionate about the inclusive definition of trapezoid that he devoted three whole blog posts to why he hates the exclusive definition of trapezoid:

http://mrchasemath.wordpress.com/2011/02/03/why-i-hate-the-definition-of-trapezoids/

http://mrchasemath.wordpress.com/2011/02/18/why-i-hate-the-definition-of-trapezoids-again/

http://mrchasemath.wordpress.com/2013/08/12/why-i-hate-the-definition-of-trapezoids-part-3/

One reason Chase states for using inclusive definitions is that it simplifies proofs:

When proving that a quadrilateral is a trapezoid, one can stop after proving just two sides are parallel. But with the exclusive definition, in order to prove that a quadrilateral is a trapezoid, you would have to prove two sides are parallel AND the other two sides are not parallel.

Regarding some of the others to whom I refer regularly, Dr. Wu uses the inclusive definition:

"A quadrilateral with at least one pair of opposite sides that are parallel is called a trapezoid. A trapezoid with two pairs of parallel opposite sides is called a parallelogram."

while Dr. Mason uses the exclusive definition:

"A trapezoid is by definition a quadrilateral with precisely one pair of parallel sides."
(emphasis Dr. M's)

So which definition should I use for trapezoid? Well, this is a Common Core blog, so the definition favored by Common Core should have priority over all other definitions. The following is a link to the information that will appear on the PARCC End-of-Year Assessment for geometry:

http://www.parcconline.org/sites/parcc/files/ES%20Table%20Geometry%20EOY%20for%20PARCC_Final.pdf

And right there in the column under "Clarifications," it reads:

i) A trapezoid is defined as “A quadrilateral with at least one pair of parallel sides.”

And that plainly settles it. The PARCC Common Core assessment uses the inclusive definition of trapezoid, and so it's my duty on a Common Core blog to use the Common Core definition. Of course, we notice that this is the definition given by PARCC -- but so far I've seen no information on what definition Smarter Balanced is using. It would be tragic if PARCC were to use one definition and Smarter Balanced the other. But as I can't say anything about Smarter Balanced, I will use the only definition that's known to be on a Common Core test, and that's the inclusive definition. The fact that this definition is already used by the U of Chicago is icing on the cake.

There is one problem with the inclusive definition of trapezoid, and that's when we try to define isosceles trapezoid. The word isosceles suggests that, just as in an isosceles triangle, an isosceles trapezoid has two equal sides -- the sides adjacent to the (parallel) bases. But in a parallelogram, where either pair of opposite sides can be considered the bases -- the sides adjacent to these bases are also equal. This would make every parallelogram an isosceles trapezoid. But this isn't desirable -- an isosceles trapezoid has several properties that parallelograms in general lack. The diagonals of an isosceles trapezoid are equal, but those of a parallelogram in general aren't. But the diagonals of a rectangle are equal. So we'd like to consider rectangles, but not parallelograms in general, to be isosceles trapezoids.

This dilemma is mentioned in the comments at one of the Chase links. It's pointed out that there are two ways out of this mess -- we may either define isosceles trapezoid in terms of symmetry, as Conway does, or we can use the U of Chicago's definition:

"A trapezoid is isosceles if and only if it has a pair of base angles equal in measure."

Some don't like this definition, because it violates linguistic purity -- the word isosceles comes from Greek, and it means "equal legs," not "equal angles." But as it turns out, it's a small price to pay to make the quadrilateral hierarchy and other theorems work out. And besides, any geometer who calls a nine-sided polygon a nonagon should just shut up about linguistic purity!

By the way, Mr. Chase has returned to posting in 2017, at least in January through April. His most recent post is about the biennial National Math Festival, held in Washington, DC.