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!)

A friendly Algebra 2 tutor note

Solve Infinite Geometric Series: how to make it click

Series skill

Algebra 2 often looks harder because there are more symbols on the page. The good move is to slow down, identify the structure, and work one clean step at a time.

Good news: most Algebra 2 mistakes are small setup mistakes. If you can name the type of problem, you are already halfway to choosing the right tool.

Start here

Decide whether you are looking at a sequence of terms or a sum of terms. Then look for the common difference or common ratio.

Watch for this

Do not use a series formula before identifying whether it is arithmetic or geometric.

1. IdentifyName the structure first: equation, function, matrix, conic, log, trig, or probability.
2. ChoosePick the matching tool before doing arithmetic. This prevents a lot of extra work.
3. CheckSubstitute, graph, or estimate to make sure the answer makes sense.

Two more tutor examples

Arithmetic sum

Example: 3 + 6 + 9 + 12

  1. There are 4 terms.
  2. Add directly or use the arithmetic-series idea.
  3. 3 + 6 + 9 + 12 = 30.

Answer: 30

Geometric pattern

Example: 2, 6, 18, 54

  1. Each term multiplies by 3.
  2. The common ratio is 3.
  3. Use that ratio for the next term.

Answer: 162

Try this quick confidence check

Try: Find the sum of 2 + 4 + 6 + 8.

Answer: 20.

When this feels steady, go back to the Algebra 2 hub and try the matching quiz or worksheet while the idea is still fresh.

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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