AMC/AIME/USA(J)MO Handout Sequences and Series in the AMC and AIME Author: For: freeman66 AoPS nikenissan Date: February 1, 2021 n n 1 A proof without words of the sum of the first n consecutive integers. \Everything comes to you in perfect time, space and sequence." { Louise Hay freeman66 and nikenissan (February 1, 2021) Sequences and Series in the AMC and AIME Contents 0 Acknowledgements 3 1 Introduction 4 2 Basics of Sequences and Series4 2.1 Definitions.................................................. 4 2.2 Summation Notation............................................ 4 3 Arithmetic Sequences and Series5 4 Geometric Sequences and Series8 5 Arithmetico-Geometric Sequence 11 6 Telescoping Series 12 7 Recursive Sequences 14 8 Problems 15 A Appendix A: List of Theorems and Definitions 18 2 freeman66 and nikenissan (February 1, 2021) Sequences and Series in the AMC and AIME §0 Acknowledgements This was made for the Art of Problem Solving Community out there! I would like to thank Evan Chen for his evan.sty code. In addition, all problems in the handout were likely from the AoPS Wiki. Art of Problem Solving Community Evan Chen's Personal Sty File freeman66's Website nikenissan's Website And Evan says he would like this here for evan.sty: Boost Software License - Version 1.0 - August 17th, 2003 Copyright (c) 2020 Evan Chen [evan at evanchen.cc] https://web.evanchen.cc/ || github.com/vEnhance He also helped with the hint formatting. Evan is a LATEXgod! And finally, please do not make any copies of this document without referencing this original one. At least cite us when you are using this document. 3 freeman66 and nikenissan (February 1, 2021) Sequences and Series in the AMC and AIME §1 Introduction Sequences and series are one of the most prevalent topics in the AMC and AIME. §2 Basics of Sequences and Series §2.1 Definitions Let's start by introducing a few ideas. Definition 2.1 (Sequence) | A sequence is an ordered list of numbers. Definition 2.2 (Partial Sum) | A partial sum is the sum of a portion of the sequence. Partial sums are sometimes called finite series. Definition 2.3 (Series) | A series is the sum of the elements in a given sequence. §2.2 Summation Notation To keep things simple we introduce summation notation. Definition 2.4 (Summation Notation) | In a summation, there is an index going from a starting point to an ending point. The given element is tested through all values from the start to the end (inclusive), and then all values of the element are summed. Summation notation is also sometimes called sigma notation. An example would be b X f(a) = f(1) + f(2) + ··· + f(b − 1) + f(b): a=1 Example 2.5 Write 1 + 2 + ::: + n in summation notation. Solution. Denote x as the index of the summation. Notice the starting point of the summation is 1 and the ending point of the summation is n: Our element is x in this scenario, thus our summation can be written as n X x : x=1 Theorem 2.6 (Distributive Property of Summations) For all real constants k, X X k · f(x) = k f(x): 4 freeman66 and nikenissan (February 1, 2021) Sequences and Series in the AMC and AIME Example 2.7 Evaluate 5 X 3 · x! : (x − 1)! x=1 Solution. By the Distributive Property of Summations, 5 5 X 3 · x! X x! = 3 : (x − 1)! (x − 1)! x=1 x=1 x! Notice (x−1)! = x: Thus, 5 5 X x! X 3 = 3 x: (x − 1)! x=1 n=1 P5 x=1 x = 1 + 2 + 3 + 4 + 5 = 15: Therefore, 5 X 3 x = 3 · 15 = 45 : x=1 Theorem 2.8 (Commutative Property of Summations) For functions f(x) and g(x), X X X (f(x) ± g(x)) = f(x) ± g(x): Summations simplify an expression a lot, and make it easier and more convenient to work with. Exercise 2.9. Simplify X X (x + 1)2 − x2: Exercise 2.10. Compute the sum 1 X k2 : 2k k=1 §3 Arithmetic Sequences and Series Definition 3.1 (Arithmetic Sequence) | An arithmetic sequence is a sequence of numbers in which each term is given by adding a fixed value to the previous term. For example, −2; 1; 4; 7; 10;::: is an arithmetic sequence because each term is three more than the previous term. In this case, 3 is called the common difference of the sequence. More formally, an arithmetic sequence an is defined recursively by a first term a1 and an = an−1 + d for n ≥ 2, where d is the common difference. Explicitly: 5 freeman66 and nikenissan (February 1, 2021) Sequences and Series in the AMC and AIME Theorem 3.2 (Terms of an Arithmetic Sequence) The nth term in an arithmetic sequence is described an = a1 + d(n − 1); th where an is the n term, a1 is the first term, and d is the difference between consecutive terms. Theorem 3.3 (Sum of an Arithmetic Sequence) The sum of the first n terms of an arithmetic sequences is n n sn = (a1 + an) = (2a1 + (n − 1)d): 2 2 Proof. We can prove this with induction, which will not be taught or explained in this handout. The following formulas are a direct consequence of the aforementioned theorems. Corollary 3.4 (Sum of First n Positive Integers) For all positive integers n, n(n + 1) 1 + 2 + 3 + ::: + n = : 2 Corollary 3.5 (Sum of First n Even Integers) For all positive integers n, 2 + 4 + ::: + 2n = n(n + 1): Corollary 3.6 (Sum of First n Odd Integers) For all positive integers n, 1 + 3 + 5 + ::: + (2n − 1) = n2: Example 3.7 (AIME I 2005/2) For each positive integer k; let Sk denote the increasing arithmetic sequence of integers whose first term is 1 and whose common difference is k: For example, S3 is the sequence 1; 4; 7; 10;:::: For how many values of k does Sk contain the term 2005? Solution. Suppose that the nth term of the sequence Sk is 2005. Then 1 + (n − 1)k = 2005 =) k(n − 1) = 2004 = 22 · 3 · 167. Because k when multiplied by an integer is equal to 2004; in order words we can say that k is a divisor/divides 2004: By the formula for the number of divisors, there are (2 + 1)(1 + 1)(1 + 1) = 12 divisors of 22 · 31 · 1671. 6 freeman66 and nikenissan (February 1, 2021) Sequences and Series in the AMC and AIME Example 3.8 (AIME II 2011/3) The degree measures of the angles in a convex 18-sided polygon form an increasing arithmetic sequence with integer values. Find the degree measure of the smallest angle. Solution. The average angle in an 18-gon is 160◦. In this arrangement, the average is equivalent to the middle, so the center two terms of the succession average to 160◦. In this manner for some certain (the grouping is expanding and hence non-consistent) whole number d, the center two terms are (160 − d)◦ and (160 + d)◦. Since the progression is 2d, the last term of the arrangement is (160 + 17d)◦, which is under 180◦, since the polygon is raised. This gives 17d < 20, so the lone appropriate positive whole number d is 1. The initial term is then (160 − 17)◦ = 143 : Example 3.9 (AIME I 2012/2) The terms of an arithmetic sequence add to 715. The first term of the sequence is increased by 1, the second term is increased by 3, the third term is increased by 5, and in general, the kth term is increased by the kth odd positive integer. The terms of the new sequence add to 836. Find the sum of the first, last, and middle terms of the original sequence. n X Solution. If the sum of the original sequence is ai then the sum of the new sequence can be expressed as i=1 n n X 2 X 2 ai + (2i − 1) = n + ai: Therefore, 836 = n + 715 =) n = 11: Now the middle term of the original i=1 i=1 715 sequence is simply the average of all the terms, or 11 = 65; and the first and last terms average to this middle term, so the desired sum is simply three times the middle term, or 195 : Example 3.10 (AMC 8 2015/18) An arithmetic sequence is a sequence in which each term after the first is obtained by adding a constant to the previous term. Each row and each column in this 5 × 5 array is an arithmetic sequence with five terms. What is the value of X? 1 25 X 17 81 (A) 21 (B) 31 (C) 36 (D) 40 (E) 42 25+1 Solution. The middle term of the first row is 2 = 13 since the middle number is just the average in an 17+81 arithmetic sequence. Similarly, the middle of the bottom row is 2 = 49. Applying this again for the middle 49+13 column, the answer is 2 = 31 . 7 freeman66 and nikenissan (February 1, 2021) Sequences and Series in the AMC and AIME Exercise 3.11 (AHSME 1969/33). Let Sn and Tn be the respective sums of the first n terms of two arithmetic series. If Sn : Tn = (7n + 1) : (4n + 27) for all n, the ratio of the eleventh term of the first series to the eleventh term of the second series is: (A) 4 : 3 (B) 3 : 2 (C) 7 : 4 (D) 78 : 71 (E) undetermined Exercise 3.12 (AIME 1999/1).