1 / 78

Exploring Sequences & Series: Patterns in Numbers

Learn about sequences and series of numbers, patterns, formulas, and graphing techniques to analyze and find terms, with interactive examples.

ebancroft
Télécharger la présentation

Exploring Sequences & Series: Patterns in Numbers

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. College Algebra Sixth Edition James StewartLothar RedlinSaleem Watson

  2. Sequences and Series 8

  3. Chapter Overview • In this chapter, we study sequences and series of numbers.

  4. Sequences and Summation Notation 8.1

  5. Sequence • Roughly speaking, a sequence is an infinite list of numbers. • The numbers in the sequence are often written as: a1, a2, a3, . . . . • The dots mean that the list continues forever. • A simple example is the sequence: 5, 10, 15, 20, 25, ...

  6. Describing Sequences • We can describe the pattern of the sequence displayed by: an = 5n

  7. Describing Sequences • You may have already thought of a different way to describe the pattern: • “You go from one number to the next by adding 5.”

  8. Describing Sequences • This natural way of describing the sequence is expressed by the recursive formulaan = an–1 + 5 starting with a1 = 5. • Try substituting n = 1, 2, 3, . . . in each of these formulas to see how they produce the numbers in the sequence.

  9. Sequences

  10. Sequence • Any ordered list of numbers can be viewed as a function whose input values are 1, 2, 3, . . . and whose output values are the numbers in the list.

  11. Sequence—Definition • A sequenceis a function f whose domain is the set of natural numbers. • The values f(1), f(2), f(3), . . ., f(n),… are called the termsof the sequence. • We usually write aninstead of the function notation f(n) for the value of the function at the number n.

  12. Sequence—Definition • So the terms of the sequence are written asa1, a2, a3, a4, …, an, … • The number a1 is called the first term, a2 is the second term, and in general anis the nth term.

  13. Sequences • Here’s a simple example: 2, 4, 6, 8, 10,… • The dots indicate that the sequence continues indefinitely. • We can write a sequence in this way when it’s clear what the subsequent terms of the sequence are. • This sequence consists of even numbers.

  14. Sequences • To be more accurate, however, we need to specify a procedure for finding allthe terms of the sequence. • This can be done by giving a formula for the nth term anof the sequence. • In this case, an = 2n and the sequence can be written as: 2, 4, 6, 8, …, 2n, …

  15. Sequences • Notice how the formula an = 2n gives all the terms of the sequence. • For instance, substituting 1, 2, 3, and 4 for n gives the first four terms: • To find the 103rd term, we use n = 103 to get: a103 = 2 · 103 = 206

  16. E.g. 1—Finding the Terms of a Sequence • Find the first five terms and the 100th term of the sequence defined by each formula.

  17. E.g. 1—Finding the Terms of a Sequence • To find the first five terms, we substitute n = 1, 2, 3, 4, and 5 in the formula for the nth term. • To find the 100th term, we substitute n = 100.

  18. E.g. 1—Finding the Terms of a Sequence • This gives the following.

  19. Terms of a Sequence • In Example 1 (d), the presence of (–1)nin the sequence has the effect of making successive terms alternately negative and positive.

  20. Graphing Sequences • It is often useful to picture a sequence by sketching its graph. • Since a sequence is a function whose domain is the natural numbers, we can draw its graph in the Cartesian plane.

  21. Graphing Sequences • For instance, the graph of the sequence is shown here.

  22. Graphing Sequences • Compare the graph of the sequence shown in Figure 1 to the graph of • shown in Figure 2.

  23. Graphing Sequences • The graph of every sequence consists of isolated points that are not connected.

  24. Analyzing Sequences • Graphing calculators are useful in analyzing sequences. • To work with sequences on a TI-83, we put the calculator in S e q mode (“sequence” mode).

  25. Analyzing Sequences • If we enter the sequence u(n) = n/(n +1) of Example 1(c), we can display the terms using the T A B L E command.

  26. Analyzing Sequences • We can also graph the sequence as shown.

  27. Terms of a Sequence • Finding patterns is an important part of mathematics. • Consider a sequence that begins 1, 4, 9, 16, … • Can you detect a pattern in these numbers? • That is, can you define a sequence whose first four terms are these numbers?

  28. Terms of a Sequence • The answer to this question seems easy. • These numbers are the squares of the numbers 1, 2, 3, 4. • Thus, the sequence we are looking for is defined by an =n2.

  29. Terms of a Sequence • However, this is not the only sequence whose first four terms are 1, 4, 9, 16. • That is, the answer to our problem is not unique (Exercise 86). • In the next example, we are interested in finding an obvioussequence whose first few terms agree with the given ones.

  30. E.g. 2—Finding the nth Term of a Sequence • Find the nth term of a sequence whose first several terms are given.

  31. Example (a) E.g. 2—Finding the nth Term • We notice that the numerators of these fractions are the odd numbers and the denominators are the even numbers. • Even numbers are of the form 2n. • Oddnumbers are of the form 2n – 1. (An odd number differs from an even number by 1.) • So, a sequence that has these numbers for its first four terms is given by:

  32. Example (b) E.g. 2—Finding the nth Term • These numbers are powers of 2 and they alternate in sign. • So, a sequence that agrees with these terms is given by: an = (–1)n2n • You should check that these formulas do indeed generate the given terms.

  33. Recursively Defined Sequences

  34. Recursive Sequence • Some sequences do not have simple defining formulas like those of the preceding example. • The nth term of a sequence may depend on some or all of the terms preceding it. • A sequence defined in this way is called recursive.

  35. E.g. 3—Finding the Terms of a Recursively Defined Sequence • A sequence defined recursively by a1 = 1 and an = 3(an–1 + 2)

  36. E.g. 3—Finding the Terms of a Recursively Defined Sequence • (a) Find the first five terms of the sequence. • (b) Use a graphing calculator to find the 20th term of the sequence.

  37. Example (a) E.g. 3—Finding Terms of Recursively Defined Sequence • The defining formula for the sequence is recursive. • It allows us to find the nth term anif we know the preceding term an–1.

  38. Example (a) E.g. 3—Finding Terms of Recursively Defined Sequence • Thus, we can find the second term from the first term, the third term from the second term, the fourth term from the third term, and so on. • Since we are given the first term a1 = 1, we can proceed as follows.

  39. Example (a) E.g. 3—Finding Terms of Recursively Defined Sequence • a2 = 3(a1 + 2) = 3(1 + 2) = 9 • a3 = 3(a2 + 2) = 3(9 + 2) = 33 • a4 = 3(a3 + 2) = 3(33 + 2) = 105 • a5 = 3(a4 + 2) = 3(105 + 2) = 321 • The first five terms of this sequence are: 1, 9, 33, 105, 321,…

  40. Example (b) E.g. 3—Finding Terms of Recursively Defined Sequence • Note that, in order to find the 20th term of the sequence in Example 3, we must first find all 19 preceding terms. • This is most easily done using a graphing calculator.

  41. Example (b) E.g. 3—Finding Terms of Recursively Defined Sequence • The first figure shows how to enter this sequence on the TI-83 calculator. • From the second, we see the 20th term is: a20 = 4,649,045,865

  42. E.g. 4—The Fibonacci Sequence • Find the first 11 terms of the sequence defined recursively by: F1 = 1, F2 = 1, and Fn = Fn–1 + Fn–2

  43. E.g. 4—The Fibonacci Sequence • To find Fn, we need to find the two preceding terms Fn–1 and Fn–2. • Since we are given F1 and F2, we proceed as follows.F3 = F2 + F1 = 1 + 1 = 2F4 = F3 + F2 = 2 + 1 = 3F5 = F4 + F3 = 3 + 2 = 5

  44. E.g. 4—The Fibonacci Sequence • It’s clear what is happening here. • Each term is simply the sum of the two terms that precede it. • So, we can easily write down as many terms as we please. • Here are the first 11 terms: 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89,…

  45. Fibonacci Sequence • The sequence in Example 4 is called the Fibonacci sequence. • It’s named after the 13th-century Italian mathematician who used it to solve a problem about the breeding of rabbits. • See Exercise 85.

  46. Fibonacci Sequence • The sequence also occurs in numerous other applications in nature.

  47. Fibonacci Sequence • In fact, so many phenomena behave like the Fibonacci sequence that one mathematical journal, the Fibonacci Quarterly, is devoted entirely to its properties.

  48. The Partial Sums of a Sequence

  49. The Partial Sums of a Sequence • In calculus, we are often interested in adding the terms of a sequence. • This leads to the following definition.

  50. Partial Sums of a Sequence—Definition • For the sequencea1, a2, a3, a4, …, an, …the partial sumsare: S1 = a1S2 = a1 + a2S3 = a1 + a2+ a3 S4 = a1 + a2 + a3 + a4 Sn = a1 + a2 + a3 + … + an

More Related