# Grades 7 & 8, Math Circles 10/11/12 October, Series & Polygonal Numbers

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1 Faculty of Mathematics Waterloo, Ontario N2L G Centre for Education in Mathematics and Computing Introduction Grades 7 & 8, Math Circles 0//2 October, 207 Series & Polygonal Numbers Mathematicians are always looking for easier and quicker ways to do mundane tasks. Adding up numbers is one such task. This week, we ll take a look at a tricky way to add consecutive natural numbers and then extend this idea to sums of both odd and even natural numbers. Sum of the First n Natural Numbers Say you were asked to find the sum of the natural numbers starting at and going all the way to 0. Pretty easy right? You just write them down in a line and add! = Neat!, you say. However, one of your classmates thinks this was way too easy. He believes that real mathematicians can do this sum all the way up to fifty. No problem, bring it on!, you say - you re no slouch! So you do the same thing as before and write out the numbers in a line so you can add them Now we ve got a slight problem - the page isn t wide enough for you to write out all the numbers! Even if it was, it d take you a while to write them all out and add them. You re amathematician.youdon thavetimeforthis!

2 Additionally, you classmate could ve asked you to do the sum all the way up to 00 instead of stopping at 50. In fact, it s rumored that the famous mathematician Carl Friedrich Gauss (Figure.) was asked to do this sum by his teacher for unruly behavior in class. Figure : Carl Friedrich Gauss So how would you do it? You don t want to be adding numbers all day! One way of thinking about this problem is to write out the series twice in the following way: Each column sums to 0 The three dots (or ellipsis) between 5 and 96 are there to tell the reader that you could keep writing out the numbers all the way from 5 to 96 but you re not going to. Interestingly, all the columns give the same sum of 0. Since there are 00 columns, and each column sums to 0, the total sum of the two rows is. You ve found the sum of the two rows above. This is series, which means that the sum of the original series is 2 the sum of the original.

4 As seen in the figure, arranging two such triangles leads to an interlocking rectangle with sides n and (n + ). By summing the two series, we mean to count the total number of unit squares in the rectangle. So, the total number of unit squares in the rectangle in Figure 2 is. In the formula we got above, notice that we didn t put a limit on how long the series could be, just that it had to end somewhere. This means that our formula applies for any series of consecutive natural numbers. Another thing to remember is that n is the number of terms in the series, including the first and last terms. So a sum from to 0 would have 0 terms, whereas a sum from to 0 would have terms. Example : For each of the following series, write down the number of terms (n) andthesumton terms: (a) (b) (c) Hint: You know how to find the sum from to n. Can you rewrite this series as a di erence of two series that start at? Sum of the First n Odd Natural Numbers So now you ve done a lot of thinking and come up with a couple of clever ways to add consecutive natural numbers really quickly. Your classmate is left speechless at the brilliance of your intellect. You can now find the sum of any series of consecutive natural numbers. So what about consecutive odd or even numbers? Can you use something from the previous section that could help us find the sum of the first n odd numbers? (2n ) 4

5 The (2n ) at the end of the above series is a general way of writing out any odd number. n is allowed to go from to wherever you want your series to end. To see how this works, try plugging in n =intoit-youshouldgetthefirstoddnumber. Theformulaallowsyou to answer the question: What s the n th odd number? Example 2: What is: (a) The st odd number? (b) The 50 th odd number? (c) The 00 th odd number? We were able to solve the previous problem visually, by arranging unit squares into triangles and then finding the total number of unit squares in an interlocking rectangle made up of two such triangles. We could do something similar here too. As before, we represent each term in the series with the corresponding number of unit squares. Interestingly, this time we re able to arrange the unit squares into a larger square (as shown in Figure ). Weirdly, the area of each big square is always a perfect square number. For example, the first big square has an area of + = 4 which means that the sum of the first 2 odd numbers is 4. Similarly, the second big square, which represents the sum of the first odd numbers has an area of. So if you had to find the sum of the first n odd numbers, your sum would be (in terms of n). That s interesting! The sum of the first n consecutive odd numbers is always a perfect square number! Another way of thinking about this is to subtract two consecutive square numbers (say 4 and 9). You always end up with an odd number! In other words, the spacing between any two consecutive perfect square numbers, is an odd number. What this means is that the square numbers alternate between odd and even numbers, just like the natural numbers! 5

6 Figure : Sum of odd numbers represented using unit squares Example : Find the sum of these series: (a) to 22 terms. (b) to 22 terms. Hint: This isn t as hard as it looks. See how the terms are related to each other? (c) to 7 terms. Hint: Same trick as in part (b). How can you get each term from the previous one? Sum of the First n Even Natural Numbers Now that we ve looked at consecutive natural numbers and odd numbers, the only thing left is the even numbers. The series we re looking at is: n Similar to what we did with the odd numbers, the last term here is 2n and n is allowed to go from all the way to whatever you like. 2n is how you d represent any even number. 6

7 Try plugging in di erent values for n like last time. n =shouldgiveyouthefirsteven number. Now that you ve handled two di erent series by arranging unit squares, how would you arrange them this time? (see Example 4(b)) If you look at the series a bit closer, you can find a general way of writing the sum to n terms. See how each term in the series is the previous one plus 2? This means that if you wanted the 4 th term of the series, you d start with 2 (the first term) and add 2 three times, giving you. What do we call repeated addition? Multiplication of course! How can you use this fact to solve our current problem? Example 4: (a) Find the sum of the series: (b) Find the sum of the series: n Hint: Remember our trick from Example? (c) Bonus: Find a way to arrange unit squares so that you get the same answer as in part (b). Polygonal Numbers As you ve probably noticed, the title of this week s lesson is called Series & Polygonal Numbers. I know what you re thinking: Where s the polygonal numbers bit? Notice how both the previous problems were solved in a pretty neat way by arranging unit squares into familiar shapes (triangles and squares). You could ve used circles instead of unit squares too. In fact, it doesn t matter what object you use to represent each term in the series. The only rule is that the objects need to be identical. Take Figure 4 (next page) for instance. We already know that it represents the sum of the first n natural numbers - so the leftmost triangle represents the sum of the first natural 7

8 number i.e, the next triangle represents the sum of the first two natural numbers i.e, + 2 = etc. Notice how each layer of the triangle represents each term in the series (they re shaded di erently for this reason). So say you wanted to find the sum of the first natural numbers (n =). Youwouldcountthenumberofcirclesinthefirstlayers starting from the top. Each of these sums i.e.,, 6, 0,... is called a triangular number because you represent them by arranging identical things in equilateral triangles. T n is the symbol we use when we want to talk about the n th triangular number. n = n = 2 n = n = 4 T = T 2 = T = 6 T 4 = 0 Figure 4: The triangular numbers We already know how to find the n th triangular number. Remember, the triangular numbers represent the sum of consecutive natural numbers (starting at ). So the formula for the n th triangular number (T n )mustbe:t n =. Similarly, Figure 5 represents the sum of the first n odd natural numbers. So the second figure from the left represents the sum of the first two odd numbers + = 4. The layers in this case are shaded as before. Each new layer represents adding a new term to our sum (just as we did before). In this case, each sum i.e., 4, 9, 6, 25, 6,... is called a square number. Youprobably already know this because square numbers pop up everywhere in geometry thanks to a particular old Greek man (Pythagoras). S n is the symbol we use when we want to talk about the n th square number. 8

9 n = n = 2 n = n = 4 S = S 2 = 4 S = 9 S 4 = 6 Figure 5: The square numbers You already know how to find the square of any given number. This means that the formula for the n th square number is: S n =. You could go even higher with pentagonal and hexagonal numbers, but they re a bit trickier to handle. In general, these numbers are called polygonal numbers. They rethe number of dots/squares/identical things needed so that they can be arranged into a regular polygon. Now you might ve noticed that I ve left out the sum of the first n even numbers. That s because these sums aren t really polygonal numbers, as you ve probably noticed from Example. You can use polygonal numbers to solve all sorts of neat problems. One of them is particularly famous and is called The Handshake Problem. It s one of the problems on this week s problem set. 9

10 Problem Set. Find the sum of all the natural numbers between 7 and. 2. Find the sum of all the odd numbers between 00 and (a) Find the th square number (b) Find the 9 th triangular number (c) Dividing a particular square number by gives 08. What s the square number? (d) Squaring a particular triangular number and adding 0 to the result gives 46. What is the triangular number? Bonus: Write your result as T n,wherenisthepositionofyour triangular number in the list of triangular numbers. Hint: You ll need to find n to answer the bonus part 4. Find the sum of the following series: to 20 terms. Hint: See Examples and You re delivering newspapers as part of your summer job and you re paid at \$ a week. Your supervisor is so impressed with your speed that she increases your weekly pay by \$2, for each of the remaining 2 weeks. How much money do you have at the end of the summer if you spend \$00 a month on ice cream and save the rest? 6. A group of n mathematicians are at a conference. How many distinct handshakes occur assuming that they re all polite and never turn down a handshake? Hint: Imagine that each person goes round handshaking everyone else. How many handshakes would the first person give? What about the second person?... 0

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