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EXAMPLE 1:
Find the sum of
the sequence:
100, 25, 5, 1, 1/5,…
Find the sum of the sequence:
100, 25, 5, 1, 1/5,…
 First, figure out if the sequence is arithmetic or
geometric.
 It is GEOMETRIC because you are MULTIPLYING by 1/5
each time.
Find the sum of the sequence:
100, 25, 5, 1, 1/5,…
 Next, write the explicit formula for this sequence. (This is not needed to
solve the problem, it is just good practice!)
 𝑎 𝑛 = 𝑎1 ∙ 𝑟 𝑛−1
 Since the first term is 100, that is plugged in for a1.
 The common ratio is 1/5
 The new equation is:
𝑎 𝑛 = 100 ∙ (
1
5
) 𝑛−1
Find the sum of the sequence:
100, 25, 5, 1, 1/5,…
 Now find the sum!
 𝑆 =
𝑎1
1−𝑟
where a1 is the first term and r is the common ratio
 𝑆 =
100
1−1/5
= 12.5
The sum of this geometric sequence is 12.5
EXAMPLE 2:
Find the sum of
the first 10 terms
in the sequence:
3, 6, 12, 24, 48, …
Find the sum of the first 10 terms in the
sequence: 3, 6, 12, 24, 48, …
 First, figure out if the sequence is arithmetic or
geometric.
 It is GEOMETRIC because you are MULTIPLYING by 2 each
time.
Find the sum of the first 10 terms in the
sequence: 3, 6, 12, 24, 48, …
 Next, write the explicit formula for this sequence. (This is not
needed to solve the problem, it is just good practice!)
 𝑎 𝑛 = 𝑎1 ∙ 𝑟 𝑛−1
Since the first term is 3, that is plugged in for a1.
The common ratio is 2
The new equation is:
𝑎 𝑛 = 3 ∙ (2) 𝑛−1
Find the sum of the first 10 terms in the
sequence: 3, 6, 12, 24, 48, …
 Now find the sum!
 𝑆 =
𝑎1(1−𝑟 𝑛)
1−𝑟
where a1 is the first term, r is the common ratio and n is the
number of terms
 𝑆 =
3(1−210)
1−2
= −3071
 The sum of this geometric sequence is -3071

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Finding the sum of a geometric sequence

  • 1. EXAMPLE 1: Find the sum of the sequence: 100, 25, 5, 1, 1/5,…
  • 2. Find the sum of the sequence: 100, 25, 5, 1, 1/5,…  First, figure out if the sequence is arithmetic or geometric.  It is GEOMETRIC because you are MULTIPLYING by 1/5 each time.
  • 3. Find the sum of the sequence: 100, 25, 5, 1, 1/5,…  Next, write the explicit formula for this sequence. (This is not needed to solve the problem, it is just good practice!)  𝑎 𝑛 = 𝑎1 ∙ 𝑟 𝑛−1  Since the first term is 100, that is plugged in for a1.  The common ratio is 1/5  The new equation is: 𝑎 𝑛 = 100 ∙ ( 1 5 ) 𝑛−1
  • 4. Find the sum of the sequence: 100, 25, 5, 1, 1/5,…  Now find the sum!  𝑆 = 𝑎1 1−𝑟 where a1 is the first term and r is the common ratio  𝑆 = 100 1−1/5 = 12.5 The sum of this geometric sequence is 12.5
  • 5. EXAMPLE 2: Find the sum of the first 10 terms in the sequence: 3, 6, 12, 24, 48, …
  • 6. Find the sum of the first 10 terms in the sequence: 3, 6, 12, 24, 48, …  First, figure out if the sequence is arithmetic or geometric.  It is GEOMETRIC because you are MULTIPLYING by 2 each time.
  • 7. Find the sum of the first 10 terms in the sequence: 3, 6, 12, 24, 48, …  Next, write the explicit formula for this sequence. (This is not needed to solve the problem, it is just good practice!)  𝑎 𝑛 = 𝑎1 ∙ 𝑟 𝑛−1 Since the first term is 3, that is plugged in for a1. The common ratio is 2 The new equation is: 𝑎 𝑛 = 3 ∙ (2) 𝑛−1
  • 8. Find the sum of the first 10 terms in the sequence: 3, 6, 12, 24, 48, …  Now find the sum!  𝑆 = 𝑎1(1−𝑟 𝑛) 1−𝑟 where a1 is the first term, r is the common ratio and n is the number of terms  𝑆 = 3(1−210) 1−2 = −3071  The sum of this geometric sequence is -3071