Arithmetic Sequence Calculator
Enter the first term and the common difference to get the n-th term, the sum of the first n terms and the full list. You can also find d and a₁ from any two known terms, or check whether a number belongs to the sequence.
The n-th term and the sum of the first n terms are calculated
How to use the Arithmetic Sequence Calculator
- Choose what you know: a₁ and d, two terms, or a value to find.
- Enter the numbers, for example a₁ = 3 and d = 4.
- Enter n, the number of terms you need.
- Press Calculate to get the n-th term and the sum with steps.
- Scroll the table to see every term and running sum, or download it as CSV.
Arithmetic sequence formulas
| What | Formula |
|---|---|
| n-th term (explicit) | aₙ = a₁ + (n − 1)d |
| Recursive form | aₙ = aₙ₋₁ + d |
| Common difference | d = aₙ − aₙ₋₁ = (a_q − aₚ)/(q − p) |
| Sum of n terms | Sₙ = n(a₁ + aₙ)/2 = n/2 · (2a₁ + (n − 1)d) |
| Position of a value x | n = (x − a₁)/d + 1 |
| Arithmetic mean of two terms | the middle term: aₖ = (aₖ₋₁ + aₖ₊₁)/2 |
Examples
| Sequence | a₁ | d | a₁₀ | S₁₀ |
|---|---|---|---|---|
| 1, 2, 3, 4, … | 1 | 1 | 10 | 55 |
| 2, 4, 6, 8, … | 2 | 2 | 20 | 110 |
| 1, 3, 5, 7, … (odd numbers) | 1 | 2 | 19 | 100 |
| 3, 7, 11, 15, … | 3 | 4 | 39 | 210 |
| 5, 10, 15, 20, … | 5 | 5 | 50 | 275 |
| 50, 45, 40, 35, … | 50 | −5 | 5 | 275 |
| 0.5, 0.75, 1, … | 0.5 | 0.25 | 2.75 | 16.25 |
Where arithmetic sequences show up
- Saving the same amount each week (without interest): the balance grows arithmetically.
- Seats in rows that each have 2 more seats than the row in front.
- Straight-line depreciation, taxi fares with a fixed charge per kilometre, and evenly spaced dates.
Frequently asked questions
What is an arithmetic sequence?
A list of numbers where each term is the previous one plus a fixed amount, the common difference d. For example 3, 7, 11, 15, … has a₁ = 3 and d = 4.
What is the formula for the n-th term?
aₙ = a₁ + (n − 1)d. For 3, 7, 11, … the 10th term is 3 + 9 × 4 = 39.
How do you find the sum of an arithmetic sequence?
Sₙ = n(a₁ + aₙ)/2, or equivalently Sₙ = n/2 · (2a₁ + (n − 1)d). The sum of the first 10 terms of 3, 7, 11, … is 10 × (3 + 39) / 2 = 210.
How do I find the common difference from two terms?
If you know aₚ and a_q, then d = (a_q − aₚ)/(q − p). With a₃ = 11 and a₁₀ = 39, d = 28/7 = 4, and a₁ = aₚ − (p − 1)d = 11 − 2 × 4 = 3.
How do I know if a number is in the sequence?
Solve a₁ + (n − 1)d = x for n: n = (x − a₁)/d + 1. The number is a term only if n comes out as a positive whole number. For 3, 7, 11, … the value 79 gives n = 20, so it is the 20th term.
What is the sum of the numbers from 1 to 100?
It is an arithmetic series with a₁ = 1, d = 1 and n = 100, so S = 100 × (1 + 100)/2 = 5,050, the result famously found by Gauss as a schoolboy.
What is the difference between an arithmetic sequence and an arithmetic series?
The sequence is the list of terms (3, 7, 11, …); the series is their sum (3 + 7 + 11 + …). This calculator gives both.