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Some Really Cool Things Happening in Pascals Triangle Jim Olsen Western Illinois University

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Outline 1.Not your typical teacher workshop – What kind of session will this be? 2.Triangular numbers 3.Ten Cool Things About Pascals Triangle 4.Tetrahedral numbers and the Twelve Days of Christmas 5.A Neat Method to find any Figurate Number

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1. What kind of session will this be? This session will be less like your typical teacher in-service workshop or math class, and more like –A play –A musical performance –Sermon –Trip to a Museum Im going to move quickly. I will continually explain things at various levels.

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Answering non-Math Questions in advance Which Illinois Learning Standards? –This relates strongly to Goal 6: Number Sense Goal 8: Algebraic Thinking Goal 10: Probability and Counting –Relates weakly to Goal 9: Geometry/Spatial Visualization

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Answering non-Math Questions in advance Q: Which Grade Level Is This? A: Grade 3 to middle school to high school to Math 355 (Combinatorics) in college

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Answering non-Math Questions in advance Q: How do I use this in my classroom? A 1 : Understand it first, then get creative. A 2 : Get the handouts and websites at the back. A 3 : Communicate with me. Id like to explore more answers to this question.

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Answering non-Math Questions in advance Q: Isnt this just math trivia? A: No. These are useful mathematical ideas (that are deep, but not hard to grasp) that can be used to solve many problems. In particular, basic number sense problems, probability problems and problems in computer science.

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What kind of session will this be? I want to look at the beauty of Pascals Triangle and improve understanding by making connections using various representations.

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2. Triangular numbers

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In General, there are Polygonal Numbers Or Figurate Numbers Example: The pentagonal numbers are 1, 5, 12, 22, …

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+3 1 +2 +9 +8 +7 +6 +5 +4 Lets Build the 9 th Triangular Number

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The Triangular Numbers are the Handshake Numbers Number of People in the Room Number of Handshakes 21 33 46 510 615 Interesting facts about Triangular Numbers Which are the number of sides and diagonals of an n-gon.

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A-B A-C A-D A-E D-E B-C B-D B-E C-D C-E Why are the handshake numbers Triangular? Lets say we have 5 people: A, B, C, D, E. Here are the handshakes: Its a Triangle !

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Q: Is there some easy way to get these numbers? A: Yes, take two copies of any triangular number and put them together…..with multi-link cubes.

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9 9+1=10 9x10 = 90 Take half. Each Triangle has 45.

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n n+1 n(n+1) Take half. Each Triangle has n(n+1)/2

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Another Cool Thing about Triangular Numbers Put any triangular number together with the next bigger (or next smaller). And you get a Square!

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Ten Cool Things About Pascals Triangle

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Characterization #1 First Definition: Get each number in a row by adding the two numbers diagonally above it (and begin and end each row with 1).

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Example: To get the 5 th element in row #7, you add the 4 th and 5 th element in row #6.

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Characterization #2 Second Definition: A Table of Combinations or Numbers of Subsets But why would the number of combinations be the same as the number of subsets?

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etc. Five Choose Two etc.

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{A, B} {A, C} {A, D} etc. Form subsets of size Two Five Choose Two {A, B, C, D, E}

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Therefore, the number of combinations of a certain size is the same as the number of subsets of that size.

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Characterization #1 and characterization #2 are equivalent, because Show me the proof

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Characterization #3 Symmetry or Now you have it, now you dont.

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Characterization #4 The total of row n = the Total Number of Subsets (from a set of size n) =2 n Why?

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Characterization #5 The Hockey Stick Principle

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Characterization #6 The first diagonal are the stick numbers. …boring, but a lead-in to…

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Characterization #7 The second diagonal are the triangular numbers. Why? Because of summing up stick numbers and the Hockey Stick Principle

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Now lets add up triangular numbers (use the hockey stick principle)…. And we get, the 12 Days of Christmas. A Tetrahedron.

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Characterization #8 The third diagonal are the tetrahedral numbers. Why? Because of summing up triangular numbers and the Hockey Stick Principle

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Tetrahedral Numbers are Cool Like Triangular Numbers Do the same things. –Find a general formula. –Add up consecutive Tetrahedral Numbers.

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Find a general formula. Use Six copies of the tetrahedron !

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Combine Two Consecutive Tetrahedrals You get a pyramid! –Wow, which is the sum of squares. –(left for you to investigate)

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Characterization #9 This is actually a table of permutations. Permutations with repetitions. Two types of objects that need to be arranged. For Example, lets say we have 2 Red tiles and 3 Blue tiles and we want to arrange all 5 tiles. How many permutations (arrangements) are there?

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For Example, lets say we have 2 Red tiles and 3 Blue tiles and we want to arrange all 5 tiles. There are 10 permutations. Note that this is also 5 choose 2. Why? Because to arrange the tiles, you need to choose 2 places for the red tiles (and fill in the rest). Or, by symmetry?… (

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Characterization #10 Imagine a ball being dropped from the top. At each pin the ball will go left or right. ** The numbers in row n are the number of different ways a ball being dropped from the top can get to that location. Row 7 >> 1 7 21 35 35 21 7 1 Imagine a pin at each location in the first n rows of Pascals Triangle (row #0 to #n-1).

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Ball dropping There are 21 different ways for the ball to drop through 7 rows of pins and end up in position 2. Why? Because position 2 is And the dropping ball got to that position by choosing to go right 2 times (and the rest left).

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Ball dropping demonstrations Physical – with actual balls. Virtual at http://www.subtangent.com/maths/flash/quincunx.swf http://www.subtangent.com/maths/flash/quincunx.swf

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Equivalently To get to Wal-Mart you have to go North 2 blocks and East 5 blocks through a grid of square blocks. There are 7 choose 2 (or 7 choose 5) ways to get to Wal-Mart. Pascals Triangle gives it to you for any size grid!

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A Neat Method to Find Any Figurate Number Number example: Lets find the 6 th pentagonal number.

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The 6 th Pentagonal Number is: Polygonal numbers always begin with 1. 1+ 5x4+ T 4 x31+20+30 = 51 Now look at the Sticks. –There are 4 sticks –and they are 5 long. Now look at the triangles! –There are 3 triangles. –and they are 4 high.

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The k th n-gonal Number is: Polygonal numbers always begin with 1. 1+ (k-1)x(n-1)+ T k-2 x(n-2) Now look at the Sticks. –There are n-1 sticks –and they are k-1 long. Now look at the triangles! –There are n-2 triangles. –and they are k-2 high.

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Jim Olsen Western Illinois University jr-olsen@wiu.edu faculty.wiu.edu/JR-Olsen/wiu/ Resources, Evaluation, Thank you.

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