Friday, January 29, 2021

Chapter 9 Review (Day 99)

Today I subbed in a high school special ed Chemistry class. This is in my first OC district. With two of the classes being co-teaching and the other having an aide, there's no need for "A Day in the Life."

(No, I won't compare the science in this class to science at the old charter school, since this is a high school class. Note that a chemistry unit appeared in eighth grade pre-NGSS science and seventh grade under the Preferred Integrated Model of NGSS.)

And the one class I actually teach is extremely small. There are 17 students enrolled in the class, but only four of them are Cohort B -- and all four of them have opted out of hybrid! The only reason there's any student in this class is that one girl in Cohort A has an arrangement to attend school everyday. (I've seen this happen with another girl in special ed at my long-term school.) She also has a one-on-one aide, and so this period has three adults for only one in-person student.

As for the two co-teaching classes, one doesn't have any students show up for in-person class, and so the resident teacher just sends me back to my room. (This class has only two students listed for in-person Cohort B anyway, so it's possible that both are absent or just late.) The other co-teaching class ends up being my largest today, with seven in-person students.

Returning to the one class that I teach myself, today's assignment is on Google Slides. It's listed as Slideshow #10 for second semester, and the first slide contains a "Do Now" Warm-Up. This suggests that this teacher assigned a slideshow with a Warm-Up each day that the class meets.

At the old charter school, I assigned Warm-Ups all the time. (My predecessor, like today's teacher, referred to Warm-Ups as "Do Now" -- and some of my students there continued to call them "Do Now" due to force of habit.) But during my long-term, I didn't assign Warm-Ups -- I wasn't sure how to do Warm-Ups during the pandemic era, and besides, the regular teacher apparently didn't have them.

But the two questions asked in today's "Do Now" are rather interesting:

1. How are you feeling about the Periodic Table? Do you feel like you understand it a little?

2. How are you doing? Anything now at home or at school you would like to share with me (this is private)?

Many teachers on either Blogger or Twitter have pointed out that personal relationships have declined during the pandemic for obvious reasons. They believe that it's their duty to maintain such relationships by asking questions like the one above, with key words like "feeling, "share," and "private."

I, of course, never gave my long-term students any Warm-Up at all, much less one where this sort of question is asked. I admit that I was never good at asking students this sort of "feelings" question, whether before or during the pandemic.

Today I sing "Palindrome Song" for the last time during Palindrome Week. While there will be two palindromic dates of the form mm-dd-yy coming up (12-11-21 and 12-22-21), neither of those falls on a school day. December 11th will be a Saturday, and December 22nd is too close to Christmas (except in New York, where schools are open until the 23rd). And so the next Palindrome Week at school will be in February 2022.

Meanwhile, as January 2021 comes to an end, I realize that no, I haven't thought much about the New Year's/Decade's Resolutions at all. It's Fiveday on the Eleven Calendar, so today's resolution is:

5. We treat people who are great at math as heroes.

But of course, it's Chemistry class, so I can't discuss any math heroes. Of course, chemists are heroes as well, especially the chemists who are working on coronavirus vaccines. Then again, I have only one girl in my in-person class and she finishes her assignment quickly, so she doesn't need extra motivation. (I don't know how hard the at-home students are working, or whether they need motivation.)

The original version of the fifth resolution was about 1955 and how students nowadays need to pay less attention to phones and more attention to class, as they did back in 1955. In sixth period I do notice a few students playing on phones, but that's the co-teaching class. If the resident teacher has nothing to say about phone use in that class, then neither do I.

So obviously, even with the new versions of the resolutions, it's still difficult to enforce these the way I did this time last year, before the pandemic. Perhaps the "one word" challenge that Shelli posted on her blog -- and my own eighth graders did the first week of the year -- makes more sense these days.

But if I replace my New Decade's Resolutions with a "one word," what word should I choose? I told my students about Shelli's "joy." but I don't wish to copy her word.

Each year, in addition to the Rapoport Calendar, I buy a Page-a-Day Word Calendar. The first word on the calendar this year was divulge. But while I do wish to reveal more information about myself as a way to enhance communication, the word has a more negative connotation -- as in divulging things that should remain secret. So it wouldn't do as a word of the year.

The second word on my calendar is homage. And not only is that a much better word, it already fits my fifth resolution. "We treat people who are great at math as heroes" becomes "We pay homage to people who are great at math" -- and chemistry, and any other field that contributes to making the world a much better place than it was last year.

And so it's settled -- my "one word" for 2021 is homage. Of course, I'll continue to follow resolutions that are easy to maintain during the pandemic, especially the seventh resolution on singing in class.

Oh, and speaking of music, today I subbed at the same school where I was from March 11th-13th -- the last three days before the schools closed. In fact, I arrived on campus expecting to be in the very same English classroom as those three fateful days, until I was sent to Chemistry instead. It's the room that had all those guitars -- and yes, the room where I lost my old songbook. Chances are that the book has been long since thrown away, but there would have been no harm in looking for it when I returned -- and even if the book is gone, I could have still played "Palindrome Song" on those guitars. (There's a possibility that I might return to that room soon after all, but I won't discuss it on the blog until then.)

Today is the day I promised that I'd discuss guitar music on the blog -- and I admit that yes, part of the reason I chose today was that the guitar room had been on my schedule all week. (Otherwise I might have posted music on Monday and then finished Arthur Benjamin on Tuesday-Friday.) I'll still write something about guitar music anyway, but it will be some time before I perform any guitar music.

Anyway, my guitar has been fixed, and I now have standard tuning (EADGBE) again. Back before my tuner was fixed, I was using EACGAE and EGCGAE tuning and suggested that these are potential tunings for an instrument fretted to 18EDL (as in the Arabic lute). But that was mainly because my D string was stuck on C, not necessarily because these are the best tunings.

I played around with several possible tunings on paper. Suppose we were to ask, what tuning, with Arabic fretting, produces the most just major and relative minor chords? For this, let's try to play as many chords related to the key of C major (C, F, G) as possible, and with true bass notes on the lower strings (so that we're playing F, not F/A). The Kite color of all strings will be either white (wa) or yellow (yo), as I suggested when I came up with earlier tunings.

Then the best tuning I was able to come up with GADACF (with D and A's yo, all others wa). To find this tuning, I started with the bass notes -- on the lowest three strings, the notes wa G, yo A, yo B, wa C, yo D, yo E, and wa F are all playable. Then I tune the higher strings in order to make a six-string G chord be playable (a possible G7 chord here is 023220).

But I'm through with alternate tunings. I finally have standard tuning on my guitar again, and I'm not getting Arabic fretting at any time soon. So from now on, I'm only looking at standard tuning.

Then again, I'm still using Mocha and EDL scales to generate my songs. And so here's a question -- given standard tuning EADGBE, how should we color the strings (wa and yo) in order to maximize the number of just major and related chords?

Well, that's easy. For starters, we'll color the G and D wa, and the B and E's yo. This produces a playable major third (wa G-yo B) and well as two playable major sixths (wa G-yo E and wa D-yo B, which can both be inverted to minor thirds).

That leaves only the A string. There are several reasons why yo A is preferable to wa A. First of all, there is no playable 18EDL scale starting on wa A in Mocha, while there are such playable scales based on wa D and wa G. And second, fingering yo A at the second fret produces yo B, and this note can be used to make a six-string G major chord (320003).

From this point, it's easy to find some other chords that are playable on the guitar. Since we're using standard EADGBE tuning, it remains only to check whether the commonly fingered chords contain notes that are colored correctly. Most importantly, notes that are an octave or perfect fifth (or fourth) apart must have the same color, or the chord will be a wolf chord (dissonant).

It's easy to check that D major (xx0232) will be colored correctly. But the usual C, A, and E chords all contain both a wa E on the D string and the open yo E string, so these aren't playable.

As for minor chords, we can add bass notes to the G and D major chords to produce an Em7 (020003) and a Bm7 (x20232) chord. There's reason that minor sevenths are playable but not pure minors -- it's possible to finger a yo string at the third fret to produce a wa note, but we can't finger a wa string to produce a yo note. Thus adding the seventh to a minor chord gives us an extra wa note for us to play on the wa strings.

OK, so our playable chords are G, D, Em7, and Bm7. It's worth continuing to investigate other chords that are playable, including those fingered at the first fret (su) or fourth fret (red/ru).

Here are all the notes that are playable at the first nine frets:

E: yo E, suyo E#, yo F#, wa G, ruyo G#, thuyo A, yo B, luyo C, wa D, yo E
B: yo B, suyo B#, yo C#, wa D, ruyo D#, thuyo E, yo F#, luyo G, wa A, yo B
G: wa G, su G#, wa A, gu Bb, ru B, thu C, wa D, lu Eb, gu F, wa G
D: wa D, su D#, wa E, gu F, ru F#, thu G, wa A, lu Bb, gu C, wa D
A: yo A, suyo A#, yo B, wa C, ruyo C#, thuyo D, yo E, luyo F, wa G, yo A
E: yo E, suyo E#, yo F#, wa G, ruyo G#, thuyo A, yo B, luyo C, wa D, yo E

(Note: here ilo 4th = A4 and tho 6th = M6. This allows luyo and thuyo to be slightly above wa.)

Written as octave-reduced intervals (using G = 1/1, since G major was our first chord), we get:

E: 5/3, 30/17, 15/8, 1/1, 15/14, 15/13, 5/4, 15/11, 3/2, 5/3
B: 5/4, 45/34, 45/32, 3/2, 45/28, 45/26, 15/8, 45/44, 9/8, 5/4
G: 1/1, 18/17, 9/8, 6/5, 9/7, 18/13, 3/2, 18/11, 9/5, 1/1
D: 3/2, 27/17, 27/16, 9/5, 27/14, 27/26, 9/8, 27/22, 27/20, 3/2
A: 10/9, 20/17, 5/4, 4/3, 10/7, 20/13, 5/3, 20/11, 1/1, 10/9
E: 5/3, 30/17, 15/8, 1/1, 15/14, 15/13, 5/4, 15/11, 3/2, 5/3

And rounded to the nearest cent (G = 0 cents), these are:

E: 884, 983, 1088, 0, 119, 248, 386, 537, 702, 884
B: 386, 485, 590, 702, 821, 950, 1088, 39, 204, 386
G: 0, 99, 204, 316, 435, 563, 702, 853, 1018, 0
D: 702, 801, 906, 1018, 1137, 65, 204, 355, 520, 702
A: 182, 281, 386, 498, 617, 746, 884, 1035, 0, 182
E: 884, 983, 1088, 0, 119, 248, 386, 537, 702, 884

Here's what all of this means for now. If I write a song in 18EDL and play chords on the guitar, then I should be playing only chords that fit a potential Arabic fretting (even though I'm playing it on my standardly fretted guitar). Thus I can play G, D, Em7, Bm7 but not C, A, E, and so on. Thus it's worth exploring more playable chords to go with these songs. (Again, this only applies to my original songs that I declare to be in an EDL scale -- established tunes, songs from Square One TV, and so on, can be played using whatever chords they need).

By the way, any note that doesn't contain "yo" in its color is playable in Mocha. This includes the full 18EDL scales playable on the G and D strings. The scale starting on D is fully in my vocal range -- the open D is sung below middle C while the D at the ninth fret is just above middle C. But the scale starting on G exceeds my range -- the G above middle C is difficult for me to reach. Therefore if I use the 18EDL scale in the future, I'm likely to use the one based on D. (This also explains what I mean when I say a string is tuned to wa D -- it means that it matches the wa D note in Mocha.)

Today is the review for Monday's Chapter 9 Test. In many ways this is a light chapter. While the modern Third Edition includes surface area in Chapter 9 (Lessons 9-9 and 9-10), my old Second Edition stops after Lesson 9-8. Then again, students who have trouble visualizing three dimensions will struggle on tomorrow's test.

By the way, this test lands on a Monday. Some teachers may question the wisdom of scheduling a test on a Monday, right after students forget over the weekend what they reviewed on Friday. In the pandemic era, a Monday test is equitable -- all students are online on Mondays, so we know that all of them are taking it at home rather than having some take it at school and some at home.

If we compare this to other teachers, notice that the science teacher at yesterday's school gave the test yesterday and today. So the students in the Tuesday/Wednesday cohort take it at home while those on the Thursday/Friday cohort take it at school. She has no way of preventing the students at home from using notes, and so she allows this to be an open-note test for the in-person kids as well.

Well, at least our students won't have any Euclid on their test. Let's return to his next proposition:

Proposition 12.
To set up a straight line at right angles to a give plane from a given point in it.


Yesterday we construct the perpendicular from a point not on the plane, and now we construct the perpendicular from a point on the plane. This construction uses yesterday's as a subroutine.

This construction is even sillier than last Friday's to perform inside a classroom. This time, we have a point A on the floor and we wish to find a point directly above it. First, we label point B on the ceiling and use yesterday's construction to find a point C directly below it. Then we construct the line through A that is parallel to BC. This last construction is the usual plane construction -- but Euclid performs this construction in the plane containing AB, and C. This plane is neither the floor nor the ceiling, but an invisible vertical plane that isn't even necessarily parallel to a wall. There is no reasonable way to perform this construction in the classroom.

And so there's no way that our students can physically perform this construction. There will be nothing like this on tomorrow's test, even though ironically, it would be easier to answer test questions about this construction than physically perform it.

Notice that the U of Chicago text doesn't actually provide the construction for drawing a parallel to a line through a point not on the line (which is a simple plane construction). The only way implied in the text to perform this construction is to make two perpendicular constructions (which we did in yesterday's post).

Many texts that teach the construction of parallel lines use copying an angle (as in corresponding or alternate interior angles). Lesson 7-10 of the Third Edition is on constructions, and duplication of an angle is given, but still no parallel lines. (I also see some DGS "constructions" mentioned there -- I wonder whether this is similar to Euclid the Game, as alluded to in last Friday's post.)

If you must, here is a modernization of the proof of Proposition 12:

Given: the segments and angles in the above construction.
Prove: AD perp. Plane P

Proof:
Statements                              Reasons
1. bla, bla, bla                        1. Given
2. AD perp. Plane P               2. Perpendicular to Parallels (spatial, last week's Prop 8)

The proof is trivial since both BC perp. Plane P and AD | | BC are true by the way Euclid constructs these lines -- in other words, they are part of "Given." If (and that's a big if) we were to prove this in the classroom, it would be more instructive to show Euclid's proof of both yesterday's and especially today's propositions directly, than to attempt to convert the proofs to two columns.

Anyway, here is the Review for Chapter 9 Test.


Thursday, January 28, 2021

Lesson 9-8: The Four-Color Problem (Day 98)

Today I subbed in an eighth grade science class. It's in my first OC district -- I've subbed at this school before, but not in this particular classroom. I will do "A Day in the Life" today as it's middle school -- and also because it's a science class. Yes, I'm returning to my old habit of comparing any middle school science class to what I taught (or failed to teach) at my old charter school.

8:30 -- This is the district where at middle schools, three periods meet in numerical order. But as it turns out, first period is the teacher's conference period.

Near the end of this period, a fire drill begins. Fortunately, there is no evacuation needed -- compare this to last week, when an evacuation occurred during my fourth period conference and students had to report to their first period class, so I suddenly had students.

9:40 -- Second period arrives. This is the first of two honors eighth grade science classes.

The students have a Go Formative quiz today. The regular teacher was hoping to set me up in her Go Formative for the day, but she can't -- that requires Formative Premium, which costs money. The same thing happened during my long-term assignment -- the regular teacher ended up giving me his password, since he doesn't have a premium account either.

The quiz is on Newton's Laws of Motion. Since these are honors students, most of them appear to finish the assessment quickly. The students have an online Equal and Opposite Reactions game to play, as well as a Bill Nye video on motion.

As I've been doing all week, I sing the Palindrome Song today. And I tell them that it's from Square One TV, a show that ended just before Bill Nye the Science Guy premiered in 1993. (Indeed, I suspect that my local TV station aired Bill Nye in the spot vacated by Square One TV.) Today's episode is actually the series finale, which first aired in 1998.

10:40 -- Second period leaves for snack break.

10:55 -- Third period arrives. This is the second of two honors eighth grade science classes.

It's just the luck of the draw that I get a conference followed by two honors classes today. There was also a sub yesterday, and he drew the three non-honors classes.

11:55 -- Third period leaves for lunch.

12:45 -- As is typical for this district, academic support begins. Students log into Zoom from home if they need extra help.

One girl does try to show up for support, but I miss her -- I only notice her comment in the Zoom chat a few minutes after she leaves. Then again, there's no guarantee I could have helped her anyway. (Of course she's not in the honors classes, so it's possible she had a question about her quiz tomorrow.)

2:00 -- Academic support ends, thus completing my day.

OK, let's compare this class to science at the old charter school. Like most science classes in California these days, this class uses the Preferred Integrated Model of the Next Generation Science Standards. I was at the charter school during the transition to the new standards, so the correct thing for me to do was teach Physical Science to eighth grade my first two years there. By my third year, the first Integrated cohort makes it to eighth grade -- but by then there's a separate science teacher. Then again, Newton's Laws count as Physical Science, and so it's likely that I would have taught this to eighth grade each year anyway.

In the classroom, I find texts for the four units taught in Integrated Science 8, albeit in Spanish. I use Google to translate the names of these four units:

1. Change over Time

2. Energy and Movement

3. Understand the Waves

4. Human Beings and Their Place in the Universe

The texts are titled California Inspire Science, published my McGraw-Hill. Notice the irony here -- for the first few years of NGSS, the district had no science texts for the new standards, and so science teachers had to cover the whole curriculum online. And now the new science texts are finally here -- right when the pandemic begins.

The first unit is all about how our planet changes over time, as well as evolution of species. The second and third units are Physical Science -- the second unit was clearly taught to eighth graders both before and after the dawn of NGSS, but the unit on waves is fairly new. And as I've mentioned on the blog before, the last unit on astronomy was once taught to sixth graders as part of pre-NGSS Earth Science.

At my old charter school, if I followed the Illinois State text, then I would have taught the four main units of Physical Science as listed in the NGSS to my eighth graders. These four units are:

1. Matter

2. Motion

3. Energy

4. Waves

That's right -- energy and movement count as one unit in Inspire but two units in Illinois State. And with motion being the second unit for both Inspire and Illinois State, I would have reached it around the same time as this district -- just after Thanksgiving.

Here's what the motion unit would have looked like at the old charter school -- including the science projects that appeared in the Illinois State text. This schedule follows the four-week cycle that I suggested in previous science posts:

Week of November 28th, 2016 -- MS-PS2-1: Apply Newton's Third Law .. (Unit 4 Science Test covering standards MS-PS1-6 and 2-1)

Week of December 5th -- Science Projects (Water Bottle Rocket, Egg Crash Box)

Week of December 12th -- MS-PS2-2: Plan an investigation to .. forces ..

Week of January 10th, 2017  -- Science Projects (create your own forces project)

Week of January 17th -- MS-PS2-3: Ask questions about .. magnetic forces (Unit 5 Science Test covering standards MS-PS2-2 and 2-3)

Week of January 23rd: Science Projects (How can electricity cause magnetism?)

Week of January 30th: MS-PS2-4: Construct and present arguments .. gravitational ..

Week of February 6th: Science Projects (How do forces act on objects? Weight on Other Planets)

Week of February 13th: MS-PS2-5: Conduct and investigation and ... fields (Unit 6 Science Test covering standards MS-PS2-4 and 2-5)

Week of February 21st: Science Projects (sandpaper friction, magnet-levitated trains)

Recall that here, a "unit" is simply my own way of numbering the four-week chunks of time during which I give two science projects and one science test.

Some of the projects listed here might not work during the pandemic, but they should have been feasible back in 2016-17. Oh, and I could have shown the students some Bill Nye videos, just like this teacher -- except that I could have sung the Bill Nye songs just as I now sing Square One TV. (The song in today's Bill Nye episode is "All in Motion.")

Lecture 12 of Prof. Arthur Benjamin's The Mathematics of Games and Puzzles: From Cards to Sudoku is called "Winning Ways -- It's Your Move." Here is a summary of the lecture:

  • In our previous lecture, we focused on the game of Chess, but of course there are many other games of strategy that lead to interesting mathematical questions. And armed with the proper insights, these games and questions can be conquered by the mathematically inclined player.
  • There are three types of games -- ones where the last player to move wins (Chess, Checkers, NIM, Cram, Domineering), ones where the winner is the first to create a structure (Tic Tac Tow, Hex, Connect Four), and ones where the winner accumulates the most (Scrabble, Go).
  • In Cram, players take turns placing dominoes on a 4 * 8 chessboard. Northrop's Game is played on the full board, where each player has eight checkers on his first rank. Players can only move forward along each file and can't pass an opponent. The last player to move is the winner.
  • NIM is a two-player game. There are piles of coins. You can take as many as you want from any one pile. The player who takes the last coin is the winner. For example, if there are three piles with 7, 5, 4 coins, you might take 4 coins from pile 1, then I take 2 coins from pile 2, and so on.
  • Charles Bouton came up with a winning strategy in the early 20th century. If there are two unequal piles and it's your turn, then take enough to the larger pile to make them equal. Then you are guaranteed to win. From a good position, all opponent moves go to a bad position.
  • If there are three or more piles, use the binary system. For example, suppose the three piles have sizes 13, 10, 6. In binary, 13 = 8 + 4 + 1, 10 = 8 + 2, 6 = 4 + 2. A good position contains an even number of each power of two -- since there's a single 1, you should take 1 from the 13 pile.
  • As it turns out, Northrop's game is just NIM in disguise. The number of spaces between checkers corresponds to the number of coins in each pile. And even Cram is a version of NIM -- in both games, the players take turns, the last move wins, and both are finite impartial games.
  • Sprangue and Grundy proved the following theorem: Every finite impartial game where whoever makes the last move wins can be transformed into a game of NIM. We can assign a NIM number, or "nimber," to each set of empty squares.
  • Chomp is an impartial game that is not equivalent to NIM. Players take turns chomping off a square on a rectangular chessboard, which eliminates all squares above it and to its right. T he player that takes the last square loses.
  • There is an existence proof that a winning strategy exists for the first player. Suppose taking the square in the upper right is a good move, then the first player should do so. If it's bad, then the second player has a good response to it -- which the first player preempts by doing so herself.
  • In a variant of Hex, red goes first. Then blue can either make the next move, or decide this one time to switch places and play as red. So red shouldn't start too strongly (such as in the center) or blue will want to switch. This is called the pie rule.
  • Indeed, I cut, you choose is a strategy to divide a cake into two pieces. If there are three or more people wanting cake, then the first player moves the knife until someone says stop. That person takes 1/3 of the cake, and then this is reduced to the two-person case.
  • Connect Four is a more complex version of Tic Tac Toe. White and Black take turns dropping checkers into a vertical 7 * 7 board. The winner is the one who places four in a row. White should begin in the center to force a win. If White doesn't, Black should start one spot closer to center.
  • Here are some Connect Four tips: appreciate the center column, create (and guard against) double threats, make forcing moves, and be patient and ponder parity.
  • Computers can beat humans in so many games. One game that has yet to be cracked by a computer is Go.
  • Games and mathematics are alike in many ways -- in both cases, we learn how to apply rules in the correct order in order to reach a goal. This is why some schools teach puzzles such as Rubik's Cube to young students.
This completes another highly enjoyable course. I've seen many other MTBoS teachers introduce puzzles to their math classes for the same reason. And today's U of Chicago lesson is, in many ways, just one big puzzle.

Lesson 9-8 of the U of Chicago text is called "The Four-Color Problem." This lesson doesn't appear anywhere in the modern Third Edition, because this is one of those "extra" lessons that we include mainly for fun.

In the past, I've mentioned several books and lectures which discuss the Four-Color Conjecture. One of these was David Kung's lectures. [2021 update: Let me snip out David Kung's lectures here, since we're now watching Arthur Benjamin's lectures.]

I wish to link to a member of MTBoS who actually teaches the Four-Color Theorem in class:

http://eatplaymath.blogspot.com/2015/10/the-four-color-theorem-and-pumpkin-time.html

Lisa Winer is the author of this post that is over five years old. She doesn't specify in what state she lives, nor does she make it easy for me to figure out what grade or class this is.

Anyway, in Winer's class, she uses the term "chromatic number" to describe the fewest number of colors required to fill in a map. The Four-Color Theorem, therefore, states that the chromatic number of any planar map is four. On a Mobius strip the maximum chromatic number is six, and of course on a torus the maximum is seven.

It's time to return to Euclid. Of course, he writes nothing about Four Colors or reflections across an axis, and so we proceed with the next proposition instead:

Proposition 11.

To draw a straight line perpendicular to a given plane from a given elevated point.


Propositions 11 and 12 are both constructions. Many of Euclid's propositions are constructions -- indeed, "The First Theorem in Euclid's Elements" (that is, Proposition I.1) featured in Lesson 4-4 is actually a construction.

Classical constructions are performed with a straightedge and compass, and David Joyce writes about the importance of actually proving constructions as theorems. But it's awkward to ask our students to perform a construction in three dimensions.

In this construction, we have a point A and a plane P, and we wish to construct the line perpendicular to P through A. How can our students do this? Is P the flat plane of the paper and A a point floating up in space?

It might be interesting to attempt Euclid's construction in the classroom. Here's how: We choose A to be a point on the ceiling and P is the plane of the floor. Thus our goal is to draw a point on the floor directly below A.

The key to this construction is to hang a rope from point A -- a rope that should be longer than the room is high. We can pull the rope at any angle and double-mark the points where the rope is touching the floor. I say "double-mark" because the point on the floor (where the rope touches) is marked (say with chalk), and then the point on the rope (where the floor touches) is marked (say with a piece of tape). The rope now can serve as a compass -- the point of the compass is at A, and the opening of the compass is set to the distance between A and the tape. The locus of all points on the floor that are the same distance from A as the point marked on the floor is a circle, and the locus of all points on a given line on the floor that are the same distance from A is a pair of points. So if we have a point (say B) drawn on a line on the floor, then we could find the unique point C on that line such that AB and AC are congruent.

All the lines on the floor can be drawn in chalk. There will be some plane constructions drawn on the floor as well, so we could use a large compass where the pencil has been replaced with chalk.

OK, so let's begin the construction. We start by drawing any line on the floor, and then we label any point on that line B. We now find C on this line exactly as given above -- we double-mark B on both the rope and floor, and then swing the rope to find C such that AB = AC.

Now we use the chalk compass to find the perpendicular bisector of BC. The midpoint is D.

Then we double-mark D with a second piece of tape, and then find the point on the last line we drew (that is, the perp. bisector of BC), to be labeled E, such that AD = AE. The second piece of tape must be higher up than the first since AD < AB, and so there's no danger of confusing which piece of tape is which.

Finally, we find the perpendicular bisector of DE. The midpoint is F. Euclid's G and H are any points on this last line -- their location doesn't matter. Only F is relevant here. AF is the desired line through A that is perpendicular to the plane of the floor, and F is directly below A.

Of course, this whole construction seems silly because of gravity. We can just hang a rope freely from A, label the point where the rope touches the ground F, and then we're done! The difference, of course, is that Euclid's three-dimensional space isn't physical space, and so there's no direction that's "favored" because of gravity or any physical force.

And so I'm not quite sure how David Joyce has in mind when he says he wants "the basics of solid geometry" to be taught better. Does he include Euclid's spatial constructions -- does he really want students to perform them? Or maybe he merely desires that students visualize the proofs in their minds while looking at the proof.

(Do you remember Euclid the Game, which is played on computers? Maybe in higher levels, players can make three-dimensional constructions that are difficult to perform in the real world!)

By the way, we can still modernize Euclid's proof:

Given: the segments and angles in the above construction.
Prove: AF perp. plane P

Proof:
Statements                              Reasons
1. bla, bla, bla                         1. Given
2. BC perp. plane (EDDA)   2. Proposition 4 from last week
(Call it plane Q. In the classroom, Q is an invisible plane parallel to a wall.)
3. GH | | BC                            3. Two Perpendiculars Theorem (planar version)
4. GH perp. plane Q               4. Perpendicular to Parallels (spatial, Tuesday's Prop 8)
5. AF in plane Q                     5. Point-Line-Plane, part f (ADF all in plane Q)
6. GH perp. AF                       6. Definition of line perpendicular to plane
7. AF perp. plane (GHDE)   7. Prop 4 (AF perp. DE is part of "Given")
8. AF perp. plane P                 8. From construction (both lines were drawn in plane P)

It might be tricky to reconcile this proof with the "rope" construction from above. In Euclid's construction, AD is designed to be perpendicular to BC, likewise AF is perp. to DE. Both of these perpendicular constructions technically occur in planes other than the floor -- yet earlier I direct you to perform perpendicular constructions on the floor -- which is the wrong plane.

But think about it -- given a point A and a line, how do we construct a line through A perpendicular to the given line? The answer is that, using the compass, we find points B and C on that line that are equidistant from A, and then find the perpendicular bisector (in that plane) of BC.

But technically, all we really need is D, the midpoint of BC. Then the line through points A and D is automatically the perpendicular bisector of BC in the correct plane. It doesn't matter how we obtain the midpoint D -- all that matters is that we find it. This includes finding the perpendicular bisector of BC in the wrong plane (that is P, the plane of the floor). This is why Euclid is able to assert and use statements like AD perp. BC in his proof, even though this isn't obvious from our ropes. (And as it happens, the perpendiculars in plane P appear later in the proof anyway, so we might as well construct these.) In the end, let's just stick to the Four-Color Theorem and two-dimensional reflections.

Today I'll post my second pandemic-friendly activity (especially after posting yesterday's making surfaces activity -- that's what the lesson was about, but it's hard to do in a pandemic). I'm not sure how to find a Four-Color activity online -- but when in doubt, just go to Desmos:

https://www.desmos.com/calculator/r1rwm2lymo

So students can create their own maps in Desmos and demonstrate how to four-color them.

I'll keep last year's worksheet on reflections -- but then again, I suspect that these could be "desmofied" as well.