Arithmetic Progression – Formula, Definition, Examples, nth term

A progression is a sequence of numbers that follow a specific pattern. An arithmetic progression (AP) is a sequence where the differences between every two consecutive terms are the same. In an arithmetic progression, there is a possibility to derive a formula for the nth term. For example, the sequence 2, 6, 10, 14, … is an arithmetic progression (AP) because it follows a pattern where each number is obtained by adding 4 to its previous term. In this sequence, nth term = 4n-2. The terms of the sequence can be obtained by substituting n=1,2,3,… in the nth term.

When n = 1, 4n-2 = 4(1)-2 = 4-2=2

When n = 2, 4n-2 = 4(2)-2 = 8-2=6

When n = 3, 4n-2 = 4(3)-2 = 12-2=10

But how do we find the nth term of a given sequence? Let us learn about arithmetic progression in this article with solved examples.

What Is Arithmetic Progression?

We can define an arithmetic progression in two ways:

    • An arithmetic progression is a sequence where the differences between every two consecutive terms are the same.
    • An arithmetic progression is a sequence where each term, except the first term, is obtained by adding a fixed number to its previous term.

For example, 1, 5, 9, 13, 17, 21, 25, 29, 33, …

Has:

    • a = 1 (the first term)
    • d = 4 (the “common difference” between terms)

In general an arithmetic sequence can be written like: {a, a+d, a+2d, a+3d, … }

Using the above example we get: {a, a+d, a+2d, a+3d, … } = {1, 1+4, 1+2×4, 1+3×4, … } = {1, 5, 9, 13, … }

Terms Used in Arithmetic Progression

From now on, we will abbreviate arithmetic progression as AP.  Here are some more AP examples:

6, 13, 20, 27, 34, . . . .

91, 81, 71, 61, 51, . . . .

π, 2π, 3π, 4π, 5π,…

-√3, −2√3, −3√3, −4√3, −5√3,…

An AP generally is shown as follows: a1, a2,a3, . . . It involves the following terminology.

First Term:

As the name suggests, the first term of an AP is the first number of the progression. It is usually represented by a1 (or) a. For example, in the sequence 6,13,20,27,34, . . . . the first term is 6. i.e., a1=6 (or) a=6.

Common Difference:

We know that an AP is a sequence where each term, except the first term, is obtained by adding a fixed number to its previous term. Here, the “fixed number” is called the “common difference” and is denoted by ‘d’ i.e., if the first term is a1, then: the second term is a1+d, the third term is a1+d+d = a1+2d, and the fourth term is a1+2d+d= a1+3d and so on. For example, in the sequence 6,13,20,27,34,. . . , each term, except the first term, is obtained by addition of 7 to its previous term. Thus, the common difference is, d=7. In general, the common difference is the difference between every two successive terms of an AP. Thus, the formula for calculating the common difference of an AP is: d = an-a{n-1}

See also  Coprime Numbers - Definition, Properties, Examples, FAQs

General Term of Arithmetic Progression (Nth Term)

The general term (or) nth term of an AP whose first term is a and the common difference is d is found by the formula an=a+(n-1)d. For example, to find the general term (or) nth term of the sequence 6,13,20,27,34,. . . ., we substitute the first term, a1=6 and the common difference, d=7 in the formula for the nth terms. Then we get, a=a+(n-1)d = 6+(n-1)7 = 6+7n-7 = 7n -1. Thus, the general term (or) nth term of this sequence is: an = 7n-1. But what is the use of finding the general term of an AP?

We know that to find a term, we can add d to its previous term. For example, if we have to find the 6th term of 6,13,20,27,34, . . . ., we can just add d=7 to the 5th term which is 34. 6th term = 5th term + 7 = 34+7 = 41 But what if we have to find the 102nd term? Isn’t it difficult to calculate it manually? In this case, we can just substitute n=102 (and also a=6 and d=7 in the formula of the nth term of an AP. Then we get:

a=a+(n-1)d

a102 = 6+(102-1)7

a102 = 6+(101)7

a102 = 713

Therefore, the 102nd term of the above sequence is 713. Thus, the general term (or) nth term of an AP is used to find any term of the AP without finding its previous term.

following table shows some AP examples and the first term, the common difference, and the general term in each casez

Arithmetic Progression First Term Common Difference

General term

nth term

AP a d an= a + (n-1)d
91,81,71,61,51, . . . . 91 -10 -10n+101
π,2π,3π,4π,5π,… π π πn

–√3, −2√3, −3√3, −4√3–,…

-√3 √3 -√3 n

Formula for Calculating Sum of Arithmetic Progression

Consider an arithmetic progression (AP) whose first term is a1 (or) a and the common difference is d.

    • The sum of first n terms of an arithmetic progression when the nth term is NOT known is S= n/2[2a+(n-1) d]
    • The sum of first n terms of an arithmetic progression when the nth term, an is known is S= n/2[a1+an]

Example:

Mr. Kevin earns $400,000 per annum and his salary increases by $50,000 per annum. Then how much does he earn at the end of the first 3 years?

Solution: The amount earned by Mr. Kevin for the first year is, a = 4,00,000. The increment per annum is, d = 50,000. We have to calculate his earnings in the 3 years. So n=3.

Substituting these values in the AP sum formula,

Sn=n/2[2a+(n-1) d]

Sn= 3/2(2(400000)+(3-1)(50000))

= 3/2 (800000+100000)

= 3/2 (900000)

= 1350000

He earned $1,350,000 in 3 years

We can get the same answer by general thinking also as follows: The annual amount earned by Mr. Kevin in the first three years is as follows. This could be calculated manually as n is a smaller value. But the above formulas are useful when n is a larger value.

Derivation of Arithmetic Progression Formula

Arithmetic progression is a progression in which every term after the first is obtained by adding a constant value, called the common difference (d). So, to find the nth term of an arithmetic progression, we know an = a1 + (n – 1)d. a1 is the first term, a1 + d is the second term, third term is a1 + 2d, and so on. For finding the sum of the arithmetic series, Sn, we start with the first term and successively add the common difference.

Sn = a1 + (a1 + d) + (a1 + 2d) + … + [a1 + (n–1)d].

We can also start with the nth term and successively subtract the common difference, so,

Sn = an + (an – d) + (a– 2d) + … + [an – (n–1)d].

Thus the sum of the arithmetic sequence could be found by either of the ways. However, on adding those two equations together, we get

Sn = a1 + (a1 + d) + (a1 + 2d) + … + [a1 + (n–1)d]

Sn = an + (an – d) + (an – 2d) + … + [an – (n–1)d]

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2Sn = (a1 + an) + (a1 + an) + (a1 + an) + … + [a1 + an].

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Notice all the d terms are added out. So,

2Sn = n (a1 + an)

Sn = [n(a1 + an)]/2

By substituting an = a1 + (n – 1)d into the last formula, we have

Sn = n/2 [a1 + a1 + (n – 1)d] …Simplifying

Sn = n/2 [2a1 + (n – 1)d].

These two formulas help us to find the sum of an arithmetic series quickly.

Arithmetic Progression Formulas List

The formulas related to arithmetic progression are as follows.

    • Common difference of an AP: d = a2 – a1.
    • nth term of an AP: an = a + (n – 1)d
    • Sum of n terms of an AP: S= n/2(2a+(n-1)d)

Arithmetic Progression Formulas

Difference Between Arithmetic Progression and Geometric Progression

The following table explains the difference between arithmetic and geometric progression:

Arithmetic progression Geometric progression
Arithmetic progression is a series in which the new term is the difference between two consecutive terms such that they have a constant value Geometric progression is defined as the series in which the new term is obtained by multiplying the two consecutive terms such that they have a constant factor
The series is identified as an arithmetic progression with the help of a common difference between consecutive terms. The series is identified as a geometric progression with the help of a common ratio between consecutive terms.
The consecutive terms vary linearly. The consecutive terms vary exponentially.
    • The common difference doesn’t need to be positive always. For example, in the sequence, 16,8,0,−8,−16,…. the common difference is: d = 8 – 16 = 0 – 8 = -8 – 0 = -16 – (-8) =… = -8

Related Topics:

FAQs on Arithmetic Progression

What Is the Arithmetic Progression Formula?

The general form of arithmetic progression general is given by a, a + d, a + 2d, a + 3d, . . . a + (n – 1)d… The formula to find the nth term is: an = a + (n – 1) × d, and the formula to find the sum of the terms is

Sn = n/2[2a + (n − 1) × d]

What Is Arithmetic Progression in Maths?

A sequence of numbers that has a common difference between any two consecutive numbers is called an arithmetic progression (A.P.). The example of A.P. is 3,6,9,12,15,18,21, …

Write the Formula To Find the Sum of N Terms of the Arithmetic Progression?

The sum of first n terms of an arithmetic progression when the nth term is NOT known is Sn = n/2[2a+(n-1)d]. The sum of first n terms of an arithmetic progression when the nth term, an is known is Sn = n/2[a1+an].

How To Find Common Difference in Arithmetic Progression?

The common difference is the value between each number in an arithmetic sequence. Therefore, you can say that the formula to find the common difference of an arithmetic sequence is: d = a(n) – a(n – 1), where a(n) is the last term in the sequence, and a(n – 1) is the previous term in the sequence.

How To Find Number of Terms in Arithmetic Progression?

The number of terms in an arithmetic progression can be simply found by the division of the difference between the last and first terms by the common difference, and then add 1.

How To Find First Term in Arithmetic Progression?

If we know ‘d'(common difference) and any term (nth term) in the progression then we can find ‘a'(first term). Example: 2,4,6,8,…….

nth term(of arithmetic progression) = a+ (n-1)d, a = first term of arithmetic progression, n = number of terms in the arithmetic progression and d = common difference.

Here, a = 2, d = 4 – 2 = 6 – 4 = 2,

If 5th term is 10 and d=2, then

a5= a + 4d; 10 = a + 4(2); 10 = a + 8; a = 2.

What Is the Difference Between Arithmetic Sequence and Arithmetic Progression?

Arithmetic Sequence/Arithmetic Series is the sum of the elements of Arithmetic Progression. Arithmetic Progression is any number of sequences within any range that gives a common difference.

How To Find the Sum of Arithmetic Progression?

To find the sum of arithmetic progression, we have to know the first term, the number of terms, and the common difference between consecutive terms. Then the formula to find the sum of an arithmetic progression is Sn = n/2[2a + (n − 1) × d] where, a = first term of arithmetic progression, n = number of terms in the arithmetic progression and d = common difference.

What Are the Types of Progressions in Maths?

There are three types of progressions in Maths. They are:

Arithmetic Progression (AP)

Geometric Progression (GP)

Harmonic Progression (HP)

Where Is Arithmetic Progression Used?

A real-life application of arithmetic progression is seen when you take a taxi. Once you ride a taxi you will be charged an initial rate and then a per mile or per kilometer charge. This shows an arithmetic sequence that for every kilometer you will be charged a certain fixed (constant) rate plus the initial rate.

What Is Nth Term in Arithmetic Progression?

The ‘nth‘ term is a formula with ‘n’ in it which enables you to find any term of a sequence without having to go up from one term to the next. ‘n’ stands for the term number so to find the 50th term we would just substitute 50 in the formula in place of ‘n’.

How Do You Solve Arithmetic Progression Problems?

The following formulas help to solve arithmetic progression problems:

    • Common difference of an AP: d = a2 – a1.
    • nth term of an AP: an = a + (n – 1)d
    • Sum of n terms of an AP: S= n/2(2a+(n-1)d)

where, a = first term of arithmetic progression, n = number of terms in the arithmetic progression, and d = common difference.

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