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9.2 Arithmetic Sequences and Partial Sums

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9.2 Arithmetic Sequences A sequence is arithmetic if the differences between consecutive terms are the same. That common difference is called d. For Ex.7, 11, 15, 19, …, 4n + 3 or 2, -3, -8, -13, …, 7 - 5n are arith. seq.’s Let’s take a look at the first example. What is d, the common difference? What is a 1 ? a 1 = 7 a 2 = = 11 a 3 = 7 + 2(4) = 15 a 4 = 7 + 3(4) = 19 Do you see a pattern? What is a 5 ? What is a n ? What is the 35th term?

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Can you come up with an equation that will give you the nth term of any arithmetic sequence? a n = a 1 + (n - 1)d Find a formula for the nth term of the arithmetic sequence whose common difference is 5 and whose 2nd term is 12. What is the 18th term of the sequence? What do we know?That d = 5 and a 2 = 12 a 2 = a 1 + (2-1)d or a 1 + dSubstitute in for a 2 and d and solve for a 1 12 = a and a 1 = 7 How now, do we find the 18th term? a 18 = a d = (5) = 92

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Find the ninth term of the arithmetic sequence whose first two terms are 2 and 9. What is the common difference, d?d = 7 Now we can find the 9th term because we know a 1 and d. Find a 9. a 9 = 2 + 8(7) = 58

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The fourth term of an arithmetic sequence is 20, and the 13th is 65. Write the first 4 terms of this sequence. Write the equations of any 4th and 13th terms of any arithmetic sequence. a 4 = a 1 + 3d a 13 = a d Now fill in what we know and use ellimination to find a 1 and d. So the first four terms are:5, 10, 15, 20 Homework 1-37 odd

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The sum of a finite arithmetic sequence Ex. Add the numbers from 1 to 100.

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Find the sum. It would take way too long to do this by hand. Using the formula just given, we can do it in seconds. First, we have to find a 1 and a 150. a 1 = a 150 = Now substitute those into the formula.

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Insert 3 arithmetic means between 4 and ____ ____ ____ 15 We need to find the common difference. Write the equation for a 5. a 5 = a 1 + 4d 15 = 4 + 4d So the sequence is?

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An auditorium has 20 rows of seats. There are 20 seats in the first row, 21 in the the second row, 22 in the third row, and so on. How many seats are there in all 20 rows? So, what do we know? Do we know a 1 ? d? Do we know how many seats are in the last row? a 20 a 20 = (1) = 39 seats in the last row. Now find S. = 590 Homework odd

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