Wednesday, October 15, 2014

Section 5-3: Conjectures (Day 49)

Section 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. 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.

In general, the U of Chicago proves theorems as soon as they are stated. But this section, Lesson 5-3, is about conjectures, just as in the Serra text. These quadrilateral properties correspond roughly to Conjectures 42 through 60, found in Chapter 6 in Serra. Meanwhile, in the U of Chicago, some of these will be proved in the very next lesson while others will wait until Chapter 7. Notice that my statement that Section 5-3 doesn't require a Parallel Postulate rings true -- in this lesson no statement is proved at all, much less proved using the Parallel Postulate!

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.

And so this lesson will not be organized as guided notes. Instead, I have prepared a special group activity to be used for this lesson. Notice that today is the day that the PSAT will be given in most high schools -- and many schools will shorten the classes in order to accommodate the exam. So with shorter classes, there might not be enough time for a regular lesson today anyway. So this activity will fit today's classes.

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.

But what does giving personal information have to do with teaching math? Recall that this activity occurs right after the students have taken the PSAT for three hours. So we expect the students to be highly resistant to doing anything academic for the rest of the day -- instead giving the tired old excuse "All of the other teachers are giving us a free period today."

And so I begin the activity with something other than geometry. It's amazing how fascinated students are in finding out personal information about the teacher -- to the extent that students who entered the class prepared to complain about having to work after the PSAT are now suddenly interested in participating in the activity.

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 33 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."

I'll let the teachers decide what prizes to award the winning team -- or teams, since I prefer to give the reward to the top two groups.

This lesson was designed to maximize student engagement from beginning to end. And the best thing about giving this game right after the PSAT is that, unlike the PSAT, here the students definitely don't lose points for guessing!

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