How to Solve Infinite Geometric Series? (+FREE Worksheet!)

Learn how to solve the Infinite Geometric Series using the following step-by-step guide and examples.

How to Solve Infinite Geometric Series? (+FREE Worksheet!)
Tutor-style math help

Solve Infinite Geometric Series: what to notice and how to work it

Series skill
Sequences list terms; series add terms. The first question is whether the pattern adds the same amount or multiplies by the same factor.

What to notice first

Identify the term number. Many mistakes happen because \(a_1\), \(a_n\), and the number of terms get mixed up.

Common student mistake

Do not use an arithmetic formula on a geometric pattern. Check differences and ratios before choosing a formula.

Key formulas and cues

\(a_n=a_1+(n-1)d\)
\(a_n=a_1r^{n-1}\)
\(S_n=\frac{n}{2}(a_1+a_n)\)
\(S_n=a_1\frac{1-r^n}{1-r}\)
591317 +4+4+4

A reliable path

  1. Compare termsLook for a common difference or common ratio.
  2. Choose term or sumDecide whether the question asks for one term or a total.
  3. Track nMake sure n is the position or number of terms the question uses.

Worked examples

Arithmetic sequence

Example: 5, 9, 13, 17, …
  1. Each term adds 4.
  2. The common difference is 4.
  3. Add 4 to continue.
Answer: \(21\)

Geometric sequence

Example: 3, 6, 12, 24, …
  1. Each term multiplies by 2.
  2. The common ratio is 2.
  3. Multiply 24 by 2.
Answer: \(48\)
Try one before moving on
Try: Find the next term: 10, 7, 4, 1, …
Answer: \(-2\). The pattern subtracts 3.
Next step: do the matching worksheet or quiz while the method is still fresh, then come back and explain the first step in your own words.

Related Topics

Step by step guide to solve Infinite Geometric Series

  • Infinite Geometric Series: The sum of a geometric series is infinite when the absolute value of the ratio is more than \(1\).
  • Infinite Geometric Series formula: \(\color{blue}{S= \sum_{i=0}^ \infty a_{i}r^i=\frac{a_{1}}{1-r}}\)

Infinite Geometric Series – Example 1:

Evaluate infinite geometric series described. \(S= \sum_{i=1}^ \infty 9^{i-1}\)

Solution:

Use this formula: \(\color{blue}{S= \sum_{i=0}^ \infty a_{i}r^i=\frac{a_{1}}{1-r}} → S= \sum_{i=1}^ \infty 9^{i-1}=\frac{1}{1-9}=\frac{1}{-8}=-\frac{1}{8}\)

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Infinite Geometric Series – Example 2:

Evaluate the infinite geometric series described. \(S= \sum_{k=1}^ \infty (\frac{1}{4})^{k-1}\)

Solution:

Use this formula: \(\color{blue}{S= \sum_{i=0}^ \infty a_{i}r^i=\frac{a_{1}}{1-r}} → S= \sum_{k=1}^ \infty (\frac{1}{4})^{k-1}=\frac{1}{1-\frac{1}{4}}=\frac{1}{\frac{3}{4}}=\frac{4}{3}\)

Infinite Geometric Series – Example 3:

Evaluate the infinite geometric series described. \(S= \sum_{i=1}^ \infty 8^{i-1}\)

Solution:

Use this formula: \(\color{blue}{S= \sum_{i=0}^ \infty a_{i}r^i=\frac{a_{1}}{1-r}} → S= \sum_{i=1}^ \infty 8^{i-1}=\frac{1}{1-8}=\frac{1}{-7}=-\frac{1}{7}\)

Infinite Geometric Series – Example 4:

Evaluate the infinite geometric series described. \(S= \sum_{k=1}^ \infty (\frac{1}{2})^{k-1}\)

Solution:

Use this formula: \(\color{blue}{S= \sum_{i=0}^ \infty a_{i}r^i=\frac{a_{1}}{1-r}} → S= \sum_{k=1}^ \infty (\frac{1}{2})^{k-1}=\frac{1}{1-\frac{1}{2}}=\frac{1}{\frac{1}{2}}=2\)

Exercises for Solving Infinite Geometric Series

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  • \(\color{blue}{Diverges}\)
  • \(\color{blue}{Converges}\)
  • \(\color{blue}{Diverges}\)
  • \(\color{blue}{Converges}\)
  • \(\color{blue}{Converges}\)
  • \(\color{blue}{Diverges}\)

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