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7.1 - 1 Sequences A sequence is a function that computes an ordered list. For example, the average person in the United States uses 100 gallons of water each day. The function defined by (n) = 100n generates the terms of the sequence 100, 200, 300, 400, 500, 600, 700,…, when n = 1, 2, 3, 4, 5, 6, 7, …. This function represents the gallons of water used by the average person after n days.

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7.1 - 2 Sequences A second example of a sequence involves investing money. If $100 is deposited into a savings account paying 5% interest compounded annually, then the function defined by g (n) = 100(1.05) n calculates the account balance after n years. The terms of the sequence are g (1), g (2), g (3), g (4), g (5), g (6), g (7), …, and can be approximated as 105, 110.25, 115.76, 121.55, 127.63, 134.01, 140.71,....

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7.1 - 3 Sequence A finite sequence is a function that has a set of natural numbers of the form {1, 2, 3, …, n} as its domain. An infinite sequence has the set of natural numbers as its domain.

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7.1 - 4 Sequences For example, the sequence of natural- number multiples of 2, 2, 4, 6, 8, 10, 12, 14, …, is infinite, but the sequence of days in June, 1, 2, 3, 4, …, 29, 30, is finite.

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7.1 - 5 Sequences Instead of using f (x) notation to indicate a sequence, it is customary to use a n, where a n = (n). The letter n is used instead of x as a reminder that n represents a natural number. The elements in the range of a sequence, called the terms of the sequence, are a 1, a 2, a 3, …. The elements of both the domain and the range of a sequence are ordered. The first term is found by letting n = 1, the second term is found by letting n = 2, and so on. The general term, or nth term, of the sequence is a n.

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7.1 - 6 Sequences These figures show graphs of (x) = 2x and a n = 2n. Notice that (x) is a continuous function, and a n is discontinuous. To graph a n, we plot points of the form (n, 2n) for n = 1, 2, 3,….

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7.1 - 7 Example 1 FINDING TERMS OF SEQUENCE Write the first five terms for each sequence. Solution a. Replacing n in with 1, 2, 3, 4, and 5 gives

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7.1 - 8 Example 1 FINDING TERMS OF SEQUENCE Write the first five terms for each sequence. Solution b. Replace n in with 1, 2, 3, 4, and 5 to obtain

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7.1 - 9 Converge and Diverge If the terms of an infinite sequence get closer and closer to some real number, the sequence is said to be convergent and to converge to that real number. For example, the sequence defined by approaches 0 as n becomes large.

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7.1 - 10 Converge and Diverge Thus a n, is a convergent sequence that converges to 0. A graph of this sequence for n = 1, 2, 3, …, 10 is shown here. The terms of a n approach the horizontal axis.

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7.1 - 11 Converge and Diverge A sequence that does not converge to any number is divergent. The terms of the sequence are 1, 4, 9, 16, 25, 36, 49, 64, 81, …. This sequence is divergent because as n becomes large, the values of do not approach a fixed number; rather, they increase without bound.

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7.1 - 12 Example 2 USING A RECURSIVE FORMULA Find the first four terms of each sequence. Solution b. This is a recursive definition. We know a 1 = 2 and a n = a n – 1 + n – 1.

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7.1 - 13 Example 3 MODELING INSECT POPULATION GROWTH Frequently the population of a particular insect does not continue to grow indefinitely. Instead, its population grows rapidly at first, and then levels off because of competition for limited resources. In one study, the behavior of the winter moth was modeled with a sequence similar to the following, where a n represents the population density in thousands per acre during year n.

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7.1 - 14 Example 3 MODELING INSECT POPULATION GROWTH Solution a. Give the table of values for n = 1, 2, 3, …, 10 Evaluate a 1, a 2, a 3, …, a 10. and

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7.1 - 15 Example 3 MODELING INSECT POPULATION GROWTH Solution a. Give the table of values for n = 1, 2, 3, …, 10 Approximate values for n = 1, 2, 3, …, 10 are shown in the table. n 12345678910 anan 12.666.2410.49.1110.29.3110.19.439.98

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7.1 - 16 b. Graph the sequence. Describe what happens to the population density. Example 3 MODELING INSECT POPULATION GROWTH Solution The graph of a sequence is a set of discrete points. Plot the points (1, 1), (2, 2.66), (3, 6.24), …,(10, 9.98), as shown here.

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7.1 - 17 Series and Summation Notation Any sequence can be used to define a series. For example, the infinite sequence defines the terms of the infinite series

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7.1 - 18 Series and Summation Notation If a sequence has terms a 1, a 2, a 3, …, then S n is defined as the sum of the first n terms. That is, The sum of the terms of a sequence, called a series, is written using summation notation. The symbol , the Greek capital letter sigma, is used to indicate a sum.

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7.1 - 19 Series A finite series is an expression of the form and an infinite series is an expression of the form The letter i is called the index of summation.

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7.1 - 20 Caution Do not confuse this use of i with the use of i to represent the imaginary unit. Other letters, such as k and j, may be used for the index of summation.

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7.1 - 21 Example 4 USING SUMMATION NOTATION Evaluate the series Solution Write each of the six terms, then evaluate the sum.

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7.1 - 22 Example 4 USING SUMMATION NOTATION Evaluate the series Solution Write each of the six terms, then evaluate the sum.

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7.1 - 23 Summation Properties If a 1, a 2, a 3, …, a n and b 1, b 2, b 3, …, b n are two sequences, and c is a constant, then for every positive integer n, (a)(b) (c)(d)

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