Mục lục bài viết

The arithmetic sequence formula is used for the calculation of the n^{th} term and sum of an arithmetic progression. The arithmetic sequence is the sequence where the common difference remains constant between any two successive terms. If we want to find any term/the sum of terms in the arithmetic sequence then we can use the arithmetic sequence formula. Let us understand the arithmetic sequence formula using solved examples.

## What is the Arithmetic Sequence Formula?

An arithmetic sequence is of the form: a, a + d, a + 2d, a + 3d,……up to n terms. The first term is a, the common difference is d, n = the number of terms. For the calculation using the arithmetic sequence formulas, first identify the first term, the number of terms and the common difference of the sequence. There are different formulas associated with an arithmetic series used to calculate the n^{th} term, sum, or the common difference of a given arithmetic sequence.

- n
^{th}term is, a_{n}= a_{1}+ (n – 1) d - Sum of n terms is, S
_{n}= (n/2) [2a_{1}+ (n – 1) d] (or) (n/2) [a_{1}+ a_{n}] - Common difference, d = a
_{n}– a_{n – 1}

In these formulas, a_{1} = the first term, d = common difference, and n = number of terms.

### Arithmetic Sequence Formula

The arithmetic sequence formulas are given as,

**Formula 1:** The **arithmetic sequence formula **to find the n^{th} term is given as,

a_{n} = a_{1} + (n – 1) d

where,

- a
_{n}= n^{th}term, - a
_{1}= first term, and - d is the common difference

**Formula 2:** The sum of first n terms in an arithmetic sequence is calculated by using one of the following formulas:

- S
_{n}= (n/2) [2a_{1}+ (n – 1) d] (when we know the first term and the common difference) - S
_{n}=(n/2) [a_{1}+ a_{n}] (when the first and the last terms)

where,

- S
_{n}= Sum of n terms, - a
_{1}= first term, - a
_{n}= n^{th}term, and - d is the common difference between the successive terms

**Formula 3:** The formula for calculating the common difference of an arithmetic sequence is given as,

d = a_{n} – a_{n – 1}

where,

- a
_{n}= n^{th}term, - a
_{n – 1}= (n – 1)^{th}term, and - d is the common difference between the successive terms

## Nth Term of Arithmetic Sequence

The n^{th} term of an arithmetic sequence a_{1}, a_{2}, a_{3}, … is given by **a _{n} = a**

_{1}

**+ (n – 1) d**. This is also known as the general term of the arithmetic sequence. This directly follows from the understanding that the arithmetic sequence a

_{1}, a

_{2}, a

_{3}, … = a

_{1}, a

_{1}+ d, a

_{1}+ 2d, a

_{1}+ 3d,… The following table shows some arithmetic sequences along with the first term, the common difference, and the n

^{th}term.

Arithmetic Sequence | First Term (a) | Common Difference (d) | n^{th} terma_{n} = a_{1} + (n – 1) d |
---|---|---|---|

80, 75, 70, 65, 60, … | 80 | -5 | 80 + (n – 1) (-5) = -5n + 85 |

π/2, π, 3π/2, 2π, …. | π/2 | π/2 | π/2 + (n – 1) (π/2) = nπ/2 |

-√2, -2√2, -3√2, -4√2, … | -√2 | -√2 | -√2 + (n – 1) (-√2) = -√2 n |

### Arithmetic Sequence Recursive Formula

The above formula for finding the n^{th }term of an arithmetic sequence is used to find any term of the sequence when the values of ‘a_{1}‘ and ‘d’ are known. There is another formula to find the n^{th} term which is called the “recursive formula of an arithmetic sequence” and is used to find a term (a_{n}) of the sequence when its previous term (a_{n-1}) and ‘d’ are known. It says

**a _{n} = a_{n-1} + d**

This formula just follows the definition of the arithmetic sequence.

**Example:** Find a_{21} of an arithmetic sequence if a_{19} = -72 and d = 7.

**Solution:**

By using the recursive formula,

a_{20} = a_{19} + d = -72 + 7 = -65

a_{21} = a_{20} + d = -65 + 7 = -58

Therefore, a_{21} = -58.

## Arithmetic Series

The sum of the arithmetic sequence formula is used to find the sum of its first n terms. Note that the sum of terms of an arithmetic sequence is known as arithmetic series. Consider an arithmetic series in which the first term is a_{1} (or ‘a’) and the common difference is d. The sum of its first n terms is denoted by S_{n}. Then

- When the n
^{th}term is NOT known: S_{n}= n/2 [2a_{1}+ (n-1) d] - When the n
^{th}term is known: S_{n}= n/2 [a_{1}+ a_{n}]

**Example**

Ms. Natalie earns $200,000 per annum and her salary increases by $25,000 per annum. Then how much does she earn at the end of the first 5 years?

**Solution:**

The amount earned by Ms. Natalie for the first year is, a = 2,00,000. The increment per annum is, d = 25,000. We have to calculate her earnings in the first 5 years. Hence n = 5. Substituting these values in the sum sum of arithmetic sequence formula,

S_{n}= n/2 [2a_{1} + (n-1) d]

⇒ S_{n} = 5/2(2(200000) + (5 – 1)(25000))

= 5/2 (400000 +100000)

= 5/2 (500000)

= 1250000

She earns $1,250,000 in 5 years. We can use this formula to be more helpful for larger values of ‘n’.

### Sum of Arithmetic Sequence

Let us take an arithmetic sequence that has its first term to be a_{1} and the common difference to be d. Then the sum of the first ‘n’ terms of the sequence is given by

S_{n} = a_{1} + (a_{1} + d) + (a_{1} + 2d) + … + a_{n} … (1)

Let us write the same sum from right to left (i.e., from the n^{th} term to the first term).

S_{n} = a_{n} + (a_{n} – d) + (a_{n} – 2d) + … + a_{1} … (2)

Adding (1) and (2), all terms with ‘d’ get canceled.

2S_{n} = (a_{1} + a_{n}) + (a_{1} + a_{n}) + (a_{1} + a_{n}) + … + (a_{1} + a_{n})

2S_{n} = n (a_{1} + a_{n})

**S _{n} = [n(a_{1} + a_{n})]/2**

By substituting a_{n} = a_{1} + (n – 1)d into the last formula, we have

S_{n} = n/2 [a_{1} + a_{1} + (n – 1)d] (or)

**S _{n} = n/2 [2a_{1} + (n – 1)d]**

Thus, we have derived both formulas for the sum of the arithmetic sequence.

## Applications of Arithmetic Sequence Formula

We use the arithmetic sequence formula every day or even every minute without even realizing it. Given below are a few real-life applications of the arithmetic sequence formula

- Stacking the cups, chairs, bowls, or a house of cards.
- Seats in a stadium or an auditorium are arranged in an arithmetic sequence.
- The seconds’ hand on the clock moves in arithmetic Sequence, and so do the minutes’ hand and the hour’s hand.
- The weeks in a month follow the arithmetic sequence, and so do the years. Each leap year can be determined by adding 4 to the previous leap year.
- The number of candles blown on a birthday increases as an arithmetic sequence every year.

## What is an Arithmetic Sequence?

An **arithmetic sequence **is defined in two ways. It is a “sequence where the differences between every two successive terms are the same” (or) In an arithmetic sequence, “every term is obtained by adding a fixed number (positive or negative or zero) to its previous term”. The following is an arithmetic sequence as every term is obtained by adding a fixed number 4 to its previous term.

### Arithmetic Sequence Example

Consider the sequence 3, 6, 9, 12, 15, …. is an arithmetic sequence because every term is obtained by adding a constant number (3) to its previous term.

Here,

- The first term, a = 3
- The common difference, d = 6 – 3 = 9 – 6 = 12 – 9 = 15 – 12 = … = 3

Thus, an arithmetic sequence can be written as a, a + d, a + 2d, a + 3d, …. Let us verify this pattern for the above example.

a, a + d, a + 2d, a + 3d, a + 4d, … = 3, 3 + 3, 3 + 2(3), 3 + 3(3), 3 + 4(3),… = 3, 6, 9, 12,15,….

A few more examples of an arithmetic sequence are:

- 5, 8, 11, 14, …
- 80, 75, 70, 65, 60, …
- π/2, π, 3π/2, 2π, ….
- -√2, -2√2, -3√2, -4√2, …

## Difference Between Arithmetic Sequence and Geometric Sequence

Here are the differences between arithmetic and geometric sequence:

Arithmetic Sequence | Geometric Sequence |
---|---|

In this, the differences between every two consecutive numbers are the same. | In this, the ratios of every two consecutive numbers are the same. |

It is identified by the first term (a) and the common difference (d). | It is identified by the first term (a) and the common ratio (r). |

There is a linear relationship between the terms. | There is an exponential relationship between the terms. |

**Important Notes on Arithmetic Sequence:**

- In arithmetic sequences, the difference between every two successive numbers is the same.
- The common difference of an arithmetic sequence a
_{1}, a_{2}, a_{3}, … is, d = a_{2}– a_{1}= a_{3}– a_{2}= … - The n
^{th}term of an arithmetic sequence is a_{n}= a_{1}+ (n−1)d. - The sum of the first n terms of an arithmetic sequence is S
_{n}= n/2[2a_{1}+ (n − 1)d]. - The common difference between arithmetic sequences can be either positive or negative or zero.

## Examples Using Arithmetic Sequence Formula

**Example 1:** Using the arithmetic sequence formula, find the 13^{th} term in the sequence 1, 5, 9, 13…

**Solution:**

To find: 13^{th} term of the given sequence.

Since the difference between consecutive terms is the same, the given sequence forms an arithmetic sequence.

a = 1, d = 4

Using arithmetic sequence formula,

a_{n} = a_{1}+ (n – 1) d

For 13^{th} term, n = 13

a_{n} = 1 + (13 – 1)4

a_{n} = 1 + (12)4

a_{n} = 1 + 48

a_{n} = 49

**Answer:** 13^{th} term in the sequence is 49.

**Example 2:** Find the first term in the arithmetic sequence where the 35^{th} term is 687 and the common difference 14.

**Solution:**

To find: The first term in the arithmetic sequence

Given: a_{n} = n^{th} term, d = 14

Using arithmetic sequence formula,

a_{n} = a_{1} + (n − 1)d

687 = a_{1} + (35 – 1)14

687 = a_{1} + (34)14

687 = a_{1} + 476

a_{1} = 211

**Answer:** The first term in the sequence is 211.

**Example 3:** Find the sum of the following arithmetic series: 3 + 7 + 11 + ……. (up to 25 terms).

**Solution:**

To find the sum of the first 25 terms of the arithmetic sequence 3, 7, 11,…….

Given: a_{1} = 3, d = 4, n = 25

The given arithmetic sequence is 3, 7, 11,….

Using the arithmetic series formula:

S_{n} = (n/2) [2a + (n – 1) d]

The sum of the first 25 terms

S_{25}=(25/2) [2 x 3 + (25 – 1) 4]

= (25/2) [6 + 24 x 4]

= 25/2 × 102

= 1275

**Answer:** The sum of the given arithmetic series is 1275.

**Question 1: **Find the 16^{th} term in arithmetic sequence 0, 2, 4, 6, 8, 10, 12, 14…..**Solution:**

The given arithmetic sequence is:

0, 2, 4, 6, 8, 10, 12, 14, …..

nth term formula is:

an = a_{1} + (n – 1)d

From the given,

a_{1} = 0 ;

n = 16 ;

d = 2

a_{16} = 0 + (16 – 1)2

a_{16} = 15 × 2

a_{16} = 30

**Question 2: Find the sum of the first 30 terms of the following sequence.****4, 7, 10, 13,….**

**Solution:**

The given arithmetic sequence is:

4, 7, 10, 13,….

Here,

a_{1} = 4

d = a_{2} – a_{1} = 7 – 4 = 3

n = 30

a_{n} = a_{1} + (n – 1)d

a_{30} = 4 + (30 – 1)3

= 4 + (29)3

= 4 + 87

= 91

Sum of the first 30 terms

S_{30} = n/2[a_{1} + a_{30}]

= (30/2)[4 + 91]

= 15 × 95

= 1425

**Question 3: What is the 25th term of the arithmetic sequence 21, 15, 9, 3, ….?**

**Solution:**

Given the arithmetic sequence is:

21, 15, 9, 3,…

Here, a_{1} = 21

d = a_{2 }– a_{1} = 15 – 21 = -6

nth term

a_{n} = a_{1} + (n – 1)d

25th term of the given sequence is:

a_{25} = a_{1} + (25 – 1)d

= 21 + 24(-6)

= 21 – 144

= -123

### Arithmetic Sequence Examples

**Example 1:** Find the n^{th }term of the arithmetic sequence -5, -7/2, -2, ….

**Solution:**The given sequence is -5, -7/2, -2, …Here, the first term is a = -5, and the common difference is, d = -(7/2) – (-5) = -2 – (-7/2) = … = 3/2.The n^{th} term of an arithmetic sequence is given bya_{n} = a_{1} + (n−1)da_{n} = -5 +(n – 1) (3/2)= -5+ (3/2)n – 3/2= 3n/2 – 13/2

**Answer:** The n^{th} term of the given arithmetic sequence is, a_{n} = 3n/2 – 13/2.

**Example 2:** Which term of the arithmetic sequence -3, -8, -13, -18,… is -248?**Solution:**The given arithmetic sequence is -3, -8, -13, -18,…The first term is, a = -3The common difference is, d = -8 – (-3) = -13 – (-8) = … = -5.It is given that the n^{th} term is, a_{n} = -248.Substitute all these values in the n^{th} term of an arithmetic sequence formula,a_{n} = a_{1} + (n−1)d

⇒ -248 = -3 + (-5)(n – 1)

⇒ -248 = -3 -5n + 5

⇒ -248 = 2 – 5n

⇒ -250 = -5n

⇒ n = 50

**Answer:** -248 is the 50^{th} term of the given sequence.

**Example 3:** Find the sum of the arithmetic sequence -3, -8, -13, -18,.., -248.

**Solution:**This sequence is the same as the one that is given in

**Example 2**.There we found that a = -3, d = -5, and n = 50.So we have to find the sum of the 50 terms of the given arithmetic series.S_{n} = n/2[a_{1} + a_{n}]S_{50} = [50 (-3 – 248)]/2 = -6275

**Answer:** The sum of the given arithmetic sequence is -6275.

## FAQs on Arithmetic Sequence Formula

### What is Arithmetic Sequence Formula in Algebra?

The **arithmetic sequence formula** refers to the formula to calculate the general term of an arithmetic sequence and the sum of the n terms of an arithmetic sequence.

- The general term of an arithmetic sequence is, a
_{n}= a_{1}+ (n – 1) d - The sum of the first ‘n’ terms of an arithmetic sequence is,
_{n}= (n/2) [2a_{1}+ (n – 1) d]

where, a_{1} = the first term and d = common difference of the sequence.

### What is n in Arithmetic Sequence Formula?

In the arithmetic sequence formula for finding the general term, a_{n} = a_{1} + (n – 1) d, ‘n’ refers to the number of the term in the given arithmetic sequence. For example, a_{2} represents the 2^{nd} term of the sequence.

### What is the Arithmetic Sequences Formula for the Sum of n Terms?

The sum of the first n terms in an arithmetic sequence is given as, S_{n} = (n/2) [2a_{1} + (n – 1) d] where S_{n} = sum of n terms, a_{1} = first term, and d is the common difference.

### What is Arithmetic Series formula?

Arithmetic series is nothing but the sum of a few or all terms of an arithmetic sequence. Thus, the arithmetic series formula is:

- S
_{n}= (n/2) [2a_{1}+ (n – 1) d] [OR] - S
_{n}= (n/2) [a_{1}+ a_{n}]

Here, a_{1} is the first term of the arithmetic series and ‘d’ is its common difference.

### How To Use the Arithmetic Sequence Formula?

To use the arithmetic sequence formula, first identify the first term (a_{1}) and the common difference (d) of the sequence. Then substitute these in the relevant formula (of n^{th} term or the sum) and simplify.

### What is the Difference Between Explicit Formula and Recursive Formula of an Arithmetic Sequence?

The explicit formula is used to find any term of an arithmetic sequence if we just know its first term and the common difference. But the recursive formula can be used to find a term only when its previous term and common difference are known.

- The explicit formula for arithmetic sequence is, a
_{n}= a_{1}+ (n – 1) d - The recursive formula for arithmetic sequence is, a
_{n}= a_{n – 1}+ d

### What is an Arithmetic Sequence in Algebra?

An arithmetic sequence in algebra is a sequence of numbers where the difference between every two consecutive terms is the same. Generally, the arithmetic sequence is written as a, a+d, a+2d, a+3d, …, where a is the first term and d is the common difference.

### What are Arithmetic Sequence Formulas?

Here are the formulas related to an arithmetic sequence where a₁ (or a) is the first term and d is a common difference:

- The common difference, d = a
_{n}– a_{n-1}. - n
^{th}term of sequence is, a_{n}= a + (n – 1)d - Sum of n terms of sequence is , S
_{n}= [n(a_{1}+ a_{n})]/2 (or) n/2 (2a + (n – 1)d)

### What is the Definition of an Arithmetic Sequence?

A sequence of numbers in which every term (except the first term) is obtained by adding a constant number to the previous term is called an **arithmetic sequence**. For example, 1, 3, 5, 7, … is an arithmetic sequence as every term is obtained by adding 2 (a constant number) to its previous term.

### How to Identify An Arithmetic Sequence?

If the difference between every two consecutive terms of a sequence is the same then it is an arithmetic sequence. For example, 3, 8, 13, 18 … is arithmetic because the consecutive terms have a fixed difference.

- 8-3 = 5
- 13-8 = 5
- 18-13 = 5 and so on.

### What is the n^{th} term of an Arithmetic Sequence?

The n^{th} term of arithmetic sequences is given by a_{n} = a + (n – 1) × d. Here ‘a’ represents the first term and ‘d’ represents the common difference.

### What is an Arithmetic Series?

An arithmetic series is a sum of an arithmetic sequence where each term is obtained by adding a fixed number to each previous term.

### What is the Arithmetic Series Formula?

The sum of the first n terms of an arithmetic sequence (arithmetic series) with the first term ‘a’ and common difference ‘d’ is denoted by Sₙ and we have two formulas to find it.

- S
_{n}= n/2[2a + (n – 1)d] - S
_{n}= n/2[a + a_{n}].

### What is the Formula to Find the Common Difference in Arithmetic sequence?

The common difference of an arithmetic sequence, as its name suggests, is the difference between every two of its successive (or consecutive) terms. The formula for finding the common difference of an arithmetic sequence is, d = a_{n} – a_{n-1}.

### How to Find n in Arithmetic Sequence?

When we have to find the number of terms (n) in arithmetic sequences, some of the information about a, d, a_{n} or S_{n} might have been given in the problem. We will just substitute the given values in the formulas of a_{n} or S_{n} and solve it for n.

### How To Find the First Term in Arithmetic sequence?

The first term of an arithmetic sequence is the number that occurs in the first position from the left. It is denoted by ‘a’. If ‘a’ is NOT given in the problem, then some information about d (or) a_{n} (or) S_{n} might be given in the problem. We will just substitute the given values in the formulas of a_{n} or S_{n} and solve it for ‘a’.

### What is the Difference Between Arithmetic Sequence and Arithmetic Series?

An arithmetic sequence is a collection of numbers in which all the differences between every two consecutive numbers are equal to a constant whereas an arithmetic series is the sum of a few or more terms of an arithmetic sequence.

### What are the Types of Sequences?

There are mainly 3 types of sequences in math. They are:

- Arithmetic sequence
- Geometric sequence
- Harmonic sequence

### What are the Applications of Arithmetic Sequence?

Here are some applications: the salary of a person which is increased by a constant amount by each year, the rent of a taxi which charges per mile, the number of fishes in a pond that increase by a constant number each month, etc.

### How to Find the n^{th} Term in Arithmetic Sequence?

Here are the steps for finding the n^{th} term of arithmetic sequences:

- Identify its first term, a
- Common difference, d
- Identify which term you want. i.e., n
- Substitute all these into the formula a
_{n}= a + (n – 1) × d.

### How to Find the Sum of n Terms of Arithmetic Sequence?

To find the sum of the first n terms of arithmetic sequences,

- Identify its first term (a)
- Common difference (d)
- Identify which term you want (n)
- Substitute all these into the formula S
_{n}= (n/2)(2a + (n – 1)d)

**FORMULAS Related Links**

✅ Arithmetic Sequence Recursive Formula ⭐️⭐️⭐️⭐️⭐

✅ Arithmetic Sequence Explicit Formula ⭐️⭐️⭐️⭐️⭐

Sum of Arithmetic Sequence Formula

✅ Sequences and Series Formulas ⭐️⭐️⭐️⭐️⭐️