jamming the death star cafeteria pool table with counterfeit coins The wonderful world of MAT 220 Dana C. Ernst, Monika Keindl, & Nándor Sieben Northern Arizona University January 24, 2017
what is mat 220? Big Picture ∙ Focus of course is on reasoning & communication through problem solving. ∙ Goal is for students to work on interesting yet challenging multi-step problems that require little background knowledge. ∙ Hope is that students will develop the habits of mind of a mathematician. ∙ The problem solving of the type in this course is a fundamental component of mathematics that receives little focused attention elsewhere in our program. 1
what is mat 220? A Hungarian-like Approach “In the Hungarian approach to learning and teaching, a strong and ex- plicit emphasis is placed on problem solving, mathematical creativity, and communication. Students learn concepts by working on problems with complexity and structure that promote perseverance and deep reflection.” “In the Hungarian approach to learning and teaching, a strong and ex- plicit emphasis is placed on problem solving, mathematical creativity, and communication. Students learn concepts by working on problems with complexity and structure that promote perseverance and deep reflection.” Ryota Matsuura (St. Olaf College & North American Director of Budapest Semesters in Mathematics Education) in On Teaching & Learning in Mathematics blog post. “In the Hungarian approach to learning and teaching, a strong and ex- plicit emphasis is placed on problem solving, mathematical creativity, and communication. Students learn concepts by working on problems with complexity and structure that promote perseverance and deep reflection.” 2
what is mat 220? Course Structure ∙ Inquiry-based learning (IBL) approach. ∙ Informal student presentations of progress on previously assigned homework problems. ∙ Exploration of alternative approaches, possible generalizations, consequences, special cases, converse. ∙ Discussion of relationships to previously assigned or solved problems. ∙ Group work focused on problems selected by the instructor. ∙ Explanation of unfamiliar mathematical concepts as needed. 3
motivation Problem Three strangers meet at a taxi stand & decide to share a cab to cut down the cost. Each has a different destination but all are heading in more-or-less the same direction. Bob is traveling 10 miles, Sally is traveling 20 miles, & Mike is traveling 30 miles. If the taxi costs $2 per mile, how much should each contribute to the total fare? 4
motivation Problem Three strangers meet at a taxi stand & decide to share a cab to cut down the cost. Each has a different destination but all are heading in more-or-less the same direction. Bob is traveling 10 miles, Sally is traveling 20 miles, & Mike is traveling 30 miles. If the taxi costs $2 per mile, how much should each contribute to the total fare? Remark In preparation for their book, The Five Elements of Effective Thinking, Burger & Starbird encountered “I don’t know” as the most common response. 4
motivation Claims 1. In addition to helping students develop procedural fluency & conceptual understanding, we must prepare them to ask & explore new questions after they leave our classrooms. Content is not enough. 2. If we really want students to be independent, inquisitive, & persistent, then we need to provide them with the means to acquire these skills. 5
motivation “Much more important than specific mathematical results are the habits of mind used by the people who create those results. […] Although it is necessary to infuse courses and curricula with modern content, what is even more important is to give students the tools they will need in order to use, understand, and even make mathe- matics that does not yet exist.” “Much more important than specific mathematical results are the habits of mind used by the people who create those results. […] Although it is necessary to infuse courses and curricula with modern content, what is even more important is to give students the tools they will need in order to use, understand, and even make mathe- matics that does not yet exist.” Cuoco, Goldenberg, & Mark in Habits of Mind: An Organizing Principle for Mathematics Curriculum. “Much more important than specific mathematical results are the habits of mind used by the people who create those results. […] Although it is necessary to infuse courses and curricula with modern content, what is even more important is to give students the tools they will need in order to use, understand, and even make mathe- matics that does not yet exist.” 6
spring 2015 (first iteration) Types of Assignments ∙ Daily Homework: Chip away at problem sequence. Focus of student presentations. ∙ Weekly Homework: Formal write-ups, revise solutions to Daily Homework. Typed using LaTeX. ∙ Coding Homework: Utilize small computer programs (Python) to solve problems. Exams ∙ 2 midterm exams and a final. ∙ Midterms consisted of an in-class part and a take-home part. ∙ Exams were a mixture of problems students had previously encountered, problems similar in nature to ones previously encountered, and new problems. 8
spring 2015 (first iteration) Typical Day ∙ Class was divided up into small groups. ∙ Each group was tasked with discussing one the problems from the Daily Homework and attempting to come to consensus on a solution. ∙ After 10–20 minutes, groups took turns sharing out. ∙ My job was to moderate discussion and guide student thinking. ∙ Students encouraged to annotate their work with colored marker pens in light of class discussion. 9
spring 2017 (current iteration) Updates ∙ Ditched Weekly and Coding assignments. ∙ Homework (formerly Daily Homework) is more or less the same as before. Chip away at problem sequence. ∙ Day-to-day format roughly the same. Need to adjust for increased class size. ∙ Replaced 2 midterms exams with 7 “quizzes”; occur every 2 weeks. ∙ Each quiz consists of problems from the previous 2 weeks and some new problems. ∙ Final exam will likely have a take-home component. 11
example problems Problem 1 Imagine a hallway with 1000 doors numbered consecutively 1 through 1000. Suppose all of the doors are closed to start with. Then some dude with nothing better to do walks down the hallway and opens all of the doors. Because the dude is still bored, he decides to close every other door starting with door number 2. Then he walks down the hall and changes (i.e., if open, he closes it; if closed, he opens it) every third door starting with door 3. Then he walks down the hall and changes every fourth door starting with door 4. He continues this way, making a total of 1000 passes down the hallway, so that on the 1000th pass, he changes door 1000. At the end of this process, which doors are open and which doors are closed? 12
example problems Problem 2 Imagine you have 25 pebbles, each occupying one square on a 5 × 5 chess board. Suppose that each pebble must move to an adjacent square by only moving up, down, left, or right. If this is possible, describe a solution. If this is impossible, explain why. 13
example problems Problem 3 An overfull prison has decided to terminate some prisoners. The jailer comes up with a game for selecting who gets terminated: 10 prisoners are to be lined up all facing the same direction. On the back of each prisoner’s head, the jailer places either a black or a red dot. Each prisoner can only see the color of the dot for all of the prisoners in front of them and the prisoners do not know how many of each color there are. The jailer tells the prisoners that if a prisoner can guess the color of the dot on the back of their head, they will live, but if they guess incorrectly, they will be terminated. The jailer will call on them in order starting at the back of the line. Before lining up the prisoners and placing the dots, the jailer allows the prisoners 5 minutes to come up with a plan that will maximize their survival. What plan can the prisoners devise that will maximize the number of prisoners that survive? 14
example problems Problem 4 Suppose you have 12 coins, all identical in appearance & weight except for one that is either heavier or lighter than the other 11 coins. Devise a procedure to identify the counterfeit coin in only 3 weighings with a two-pan scale. What about n coins? 15
example problems Problem 5 I have 10 sticks in my bag. The length of each stick is an integer. No matter which 3 sticks I try to use, I cannot make a triangle out of those sticks. What is the minimum length of the longest stick? 16
example problems Problem 6 Three actors and their three agents want to cross a river in a boat that is capable of holding only two people at a time, under the constraint that no actor can be in the presence of another agent unless their own agent is also present, because each agent is worried their rivals will poach their client. How should they cross the river with the least amount of rowing? 17
example problems Problem 7 We have two strings of pyrotechnic fuse. The strings do not look homogeneous in thickness but both of them have a label saying 4 minutes. So we can assume that it takes 4 minutes to burn through either of these fuses. How can we measure a one minute interval? 18
example problems Problem 8 Take 15 poker chips or coins, divide into any number of piles with any number of chips in each pile. Arrange piles in adjacent columns. Take the top chip off every column and make a new column to the left. Repeat forever. What happens? Make conjectures about what happens when we change the number of chips. 19
example problems Problem 9 Four red ants and two black ants are walking along the edge of a one meter stick. The four red ants, called Albert, Bart, Debbie, and Edith, are all walking from left to right, and the two black ants, Cindy and Fred, are walking from right to left. The ants always walk at exactly one centimeter per second. Whenever they bump into another ant, they immediately turn around and walk in the other direction. And whenever they get to the end of a stick, they fall off. Albert starts at the left hand end of the stick, while Bart starts 20.2 cm from the left, Debbie is at 38.7cm, Edith is at 64.9cm and Fred is at 81.8cm. Chuck’s position is not known—all we know is that he starts somewhere between Bart and Debbie. Which ant is the last to fall off the stick? And how long will it be before he or she does fall off? 20
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