Geometric Sequence
A geometric sequence or geometric progression is a sequence of numbers such that the ratios between successive terms is a constant r, called the common ratio.
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3, 6, 12, 24, 48, ...
6/3 = 2.
12/6 = 2.
24/12 = 2.
48/24 = 2.
r = 2.
Nth Term of a Geometric Sequence
1If the 1st term is known.
an = a1 · rn-1
3, 6, 12, 24, 48, ..
an = 3 · 2n-1 = 3 · 2n · 2-1 = (3/2)· 2n
2If the value that occupies any other term of the sequence is known.
an = ak · rn-k
a4= 24, k=4 and r=2.
an = a4 · rn-4
an = 24· 2n-4= (24/16)· 2n = (3/2) · 2n
Geometric Series
A geometric series is the sum of a geometric sequence.
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Compute the sum of the first 5 terms of the sequence: 3, 6, 12, 24, 48, ...
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Infinite Geometric Series
If −1 < r < 1 the infinite geometric series converges to a specific value:
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Calculate the sum of the terms of the sequence:
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Product
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Calculate the product of the first 5 terms of the sequence: 3, 6, 12, 24, 48, ...
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