Activity 8.2.4. The formulas we have derived for an infinite geometric series and its partial sum have assumed we begin indexing the sums at n = 0. If instead we have a sum that does not begin at n = 0, we can factor out common terms and use the established formulas. This process is illustrated in the examples in this activity. a. Consider the sum *********** 2 3 L® (²) * = m ( ² ) + ~ ( ² ) ² + (2) (²3) * + .... (2) (2) (2) k=1 Remove the common factor of (2) (3) from each term and hence find the sum of the series. b. Next let a and r be real numbers with -1 < r < 1. Consider the sum ∞ k=3 ark ar³ + ar¹ + ar5 +…... = Remove the common factor of ar³ from each term and find the sum of the series. k=n c. Finally, we consider the most general case. Let a and r be real numbers with −1 < r < 1, let n be a positive integer, and consider the sum ark = arn +arn+1 +arn+² +.... Remove the common factor of arn from each term to find the sum of the series.

Advanced Engineering Mathematics
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ISBN:9780470458365
Author:Erwin Kreyszig
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Chapter2: Second-order Linear Odes
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Activity 8.2.4. The formulas we have derived for an infinite geometric series
and its partial sum have assumed we begin indexing the sums at n = = 0. If
instead we have a sum that does not begin at n = 0, we can factor out common
terms and use the established formulas. This process is illustrated in the
examples in this activity.
a. Consider the sum
∞
k
2
3
Σ²) ( ³² ) * = (²) ( ² ) + (~² ( ² ) ²
(2)
) + ( ( ² ) ² +
(2)
(2)
3
k=1
3
Remove the common factor of (2) (3) from each term and hence find the
sum of the series.
b. Next let a and r be real numbers with −1 < r < 1. Consider the sum
∞
Σark = ar³ + ar¹ + ar³ +
k=3
Remove the common factor of ar³ from each term and find the sum of
the series.
c. Finally, we consider the most general case. Let a and r be real numbers
with −1 < r < 1, let n be a positive integer, and consider the sum
∞
k=n
ark = ar + ar
₂n+1
₂n+2
+ ar'
+ ..
Remove the common factor of ar" from each term to find the sum of the
series.
Transcribed Image Text:Activity 8.2.4. The formulas we have derived for an infinite geometric series and its partial sum have assumed we begin indexing the sums at n = = 0. If instead we have a sum that does not begin at n = 0, we can factor out common terms and use the established formulas. This process is illustrated in the examples in this activity. a. Consider the sum ∞ k 2 3 Σ²) ( ³² ) * = (²) ( ² ) + (~² ( ² ) ² (2) ) + ( ( ² ) ² + (2) (2) 3 k=1 3 Remove the common factor of (2) (3) from each term and hence find the sum of the series. b. Next let a and r be real numbers with −1 < r < 1. Consider the sum ∞ Σark = ar³ + ar¹ + ar³ + k=3 Remove the common factor of ar³ from each term and find the sum of the series. c. Finally, we consider the most general case. Let a and r be real numbers with −1 < r < 1, let n be a positive integer, and consider the sum ∞ k=n ark = ar + ar ₂n+1 ₂n+2 + ar' + .. Remove the common factor of ar" from each term to find the sum of the series.
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