How to Solve Arithmetic Series

How to Solve Arithmetic Series

An arithmetic series is a sequence of numbers in which each term is the sum of the previous term and a constant common difference. The general formula for an arithmetic series is:

Tutor-style math help

Solve Arithmetic 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.
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\(a + (a + d) + (a + 2d) + … + (a + (n-1)d)\) \(=\) \(\frac{n(a + (a + (n-1)d))}{2}\)

Where:

  • \(a\) is the first term of the series
  • \(d\) is the common difference
  • \(n\) is the number of terms in the series

Related Topics

Step-by-step to Solve Arithmetic Series

To solve an arithmetic series, you can use the following steps:

  1. Identify the first term \((a)\), the common difference \((d)\), and the number of terms \((n)\) in the series.
  2. Use the general formula of an arithmetic series to find the sum of the series: \(\frac{n(a + (a + (n-1)d))}{2}\)
  3. If you want to find a specific term of the series, you can use the formula: \(a + (n-1)d\)

For example, if you have an arithmetic series with \(a = 2\), \(d = 3\) and \(n = 4\), the sum of the series is: \(\frac{n(a + (a + (n-1)d)}{2}\) \(=\) \(\frac{4(2 + (2 + (4-1)3))}{2}\) \(=\) \(\frac{4(2 + 2 + 9)}{2}\) \(=\) \(\frac{4(13)}{2} = 52\)

If you want to find the 4th term of the series, you can use the formula: \(a + (n-1)d = 2 + (4-1)3 = 2 + 3 = 5\)

Arithmetic series are widely used in various fields such as finance, science, statistics, and many more.

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