11-5 geometric series - poudre school districtstaffweb.psdschools.org/kemotich/mrs_motichka/algebra...
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115 Geometric Series
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11‑5 Geometric Series
Objective: Write and evaluate geometric series.
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Check Skills You'll Need
Find each sum or difference.
1. 100 + 50 + 25 + 25/2 + 25/4 2. 3 + 9 + 27 + 81
3. 2 + 4 8 + 16 32 4. 5 10 20 40
Simplify each fraction.
5. 6. 7.1 1/51/3
11 1/4
1/2 1/31/4
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A Geometric Series is the expression for the sum of the terms of a geometric sequence.
There are two types of geometric series:
Finite (it stops) Infinite (has no end)
2 + 4 + 8 + 16 2 + 4 + 8 + 16 ...
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As with arithmetic series, you can use a formula to evaluate a finite geometric series.
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Example #1: Using the Geometric Series Formula
Use the formula to evaluate the series 3 + 6 + 12 + 24 + 48 + 96.
What is the first term (a1)?
How many terms are there (n)?
What is the common ratio (r)?
Plug the numbers into the formula: Sn = a1(1 rn) 1 r
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Evaluate each series.
a. 45 + 135 405 + 1215 3645
b. 1/3 + 1/9 + 1/27 + 1/81
Sn = a1(1 rn) 1 r
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In some cases, you can evaluate an infinite geometric series.
When |r| < 1, the series converges, or gets closer and closer to the sum, S.
When |r| ≥ 1, the series diverges, or approaches no limit.
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Example #2 Determining Divergence and Convergence
Decide whether each infinite geometric series diverges or converges. State whether the series has a sum.
a. 1 1/3 + 1/9 ... b.
5(2)n1n = 1
∞
Since |r| < 1, the seriesconverges, and the series has a sum.
Since |r| ≥ 1, the seriesdiverges, and the series does not have a sum.
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As with a finite geometric series, you can use a formula to evaluate an infinite geometric series if |r| < 1.
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Example #3 Using the Infinite Geometric Series Formula
Evaluate the infinite geometric series 1 + 1/2 + 1/4 + 1/8...
What is the first term (a1)?
What is the common ratio (r)?
Plug the numbers into the formula:
S = a1 1 r
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Recap:
What is the formula for the sum of a FINITE geometric series?
What is the formula for the sum of an INFINITE geometric series that converges?
Why is there no formula for an INFINITE geometric series that diverges?
Sn = a1(1 rn) 1 r
S = a1 1 r
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Homework
p. 628 #228 even, 3237