How to Find the General Term of Sequences

Updated on January 12, 2020

Ray is a Licensed Civil Engineer and specializes in Structural Engineering. He loves to write anything about education.

What Is a Sequence?

A sequence is a function whose domain is an ordered list of numbers. These numbers are positive integers starting with 1. Sometimes, people mistakenly use the terms series and sequence. A sequence is a set of positive integers while series is the sum of these positive integers. The denotation for the terms in a sequence is:

a₁, a₂, a₃, a₄, a_n, . . .

Finding the nth term of a sequence is easy given a general equation. But doing it the other way around is a struggle. Finding a general equation for a given sequence requires a lot of thinking and practice but, learning the specific rule guides you in discovering the general equation. In this article, you will learn how to induce the patterns of sequences and write the general term when given the first few terms. There is a step-by-step guide for you to follow and understand the process and provide you with clear and correct computations.

General Term of Arithmetic and Geometric Series | Source

What Is an Arithmetic Sequence?

An arithmetic series is a series of ordered numbers with a constant difference. In an arithmetic sequence, you will observe that each pair of consecutive terms differs by the same amount. For example, here are the first five terms of the series.

3, 8, 13, 18, 23

Do you notice a special pattern? It is obvious that each number after the first is five more than the preceding term. Meaning, the common difference of the sequence is five. Usually, the formula for the nth term of an arithmetic sequence whose first term is a₁ and whose common difference is d is displayed below.

a_n = a₁ + (n - 1) d

Steps in Finding the General Formula of Arithmetic and Geometric Sequences

1. Create a table with headings n and a_n where n denotes the set of consecutive positive integers, and a_n represents the term corresponding to the positive integers. You may pick only the first five terms of the sequence. For example, tabulate the series 5, 10, 15, 20, 25, . . .

n	an
1	5
2	10
3	15
4	20
5	25

2. Solve the first common difference of a. Consider the solution as a tree diagram. There are two conditions for this step. This process applies only to sequences whose nature are either linear or quadratic.

Condition 1: If the first common difference is a constant, use the linear equation ax + b = 0 in finding the general term of the sequence.

a. Pick two pairs of numbers from the table and form two equations. The value of n from the table corresponds to the x in the linear equation, and the value of a_n corresponds to the 0 in the linear equation.

a(n) + b = a_n

b. After forming the two equations, calculate a and b using the subtraction method.

c. Substitute a and b to the general term.

d. Check if the general term is correct by substituting the values in the general equation. If the general term does not meet the sequence, there is an error with your calculations.

Condition 2: If the first difference is not constant and the second difference is constant, use the quadratic equation ax² + b(x) + c = 0.

a. Pick three pairs of numbers from the table and form three equations. The value of n from the table corresponds to the x in the linear equation, and the value of an corresponds to the 0 in the linear equation.

an² + b(n) + c = a_n

b. After forming the three equations, calculate a, b, and c using the subtraction method.

c. Substitute a, b, and c to the general term.

d. Check if the general term is correct by substituting the values in the general equation. If the general term does not meet the sequence, there is an error with your calculations.

Finding the General Term of a Sequence | Source

Problem 1: General Term of an Arithmetic Sequence Using Condition 1

Find the general term of the sequence 7, 9, 11, 13, 15, 17,. . .

Solution

a. Create a table of a_n and n values.

n	an
1	7
2	9
3	11
4	13
5	15
6	17

b. Take the first difference of a_n.

First Difference of Arithmetic Series | Source

c. The constant difference is 2. Since the first difference is a constant, therefore the general term of the given sequence is linear. Pick two sets of values from the table and form two equations.

General Equation:

an + b = a_n

Equation 1:

at n = 1, a₁ = 7

a (1) + b = 7

a + b = 7

Equation 2:

at n = 2 , a₂ = 9

a (2) + b = 9

2a + b = 9

d. Subtract the two equations.

(2a + b = 9) - (a + b = 7)

a = 2

e. Substitute the value of a = 2 in equation 1.

a + b = 7

2 + b = 7

b = 7 - 2

b = 5

f. Substitute the values a = 2 and b = 5 in the general equation.

an + b = a_n

2n + 5 = a_n

g. Check the general term by substituting the values into the equation.

a_n = 2n + 5

a₁= 2(1) + 5 = 7

a₂= 2(2) + 5 = 9

a₃= 2(3) + 5 = 11

a₄= 2(4) + 5 = 13

a₅= 2(5) + 5 = 15

a₆= 2(6) + 5 = 17

Therefore, the general term of the sequence is:

a_n = 2n + 5

Problem 2: General Term of Arithmetic Sequence Using Condition 2

Find the general term of the sequence 2, 3, 5, 8, 12, 17, 23, 30,. . .

Solution

a. Create a table of a_n and n values.

n	an
1	2
2	3
3	5
4	8
5	12
6	17
7	23
8	30

b. Take the first difference of a_n. If the first difference of a_n is not constant, take the second.

First and Second Difference of the Arithmetic Series | Source

c. The second difference is 1. Since the second difference is a constant, therefore the general term of the given sequence is quadratic. Pick three sets of values from the table and form three equations.

General Equation:

an² + b(n) + c = a_n

Equation 1:

at n = 1, a₁ = 2

a (1) + b (1) + c = 2

a + b + c = 2

Equation 2:

at n = 2, a₂ = 3

a (2)² + b (2) + c = 3

4a + 2b + c = 3

Equation 3:

at n = 3, a₂ = 5

a (3)² + b (3) + c = 5

9a + 3b + c = 5

d. Subtract the three equations.

Equation 2 - Equation 1: (4a + 2b + c = 3) - (a + b + c = 2)

Equation 2 - Equation 1: 3a + b = 1

Equation 3 - Equation 2: (9a + 3b + c = 5) - (4a + 2b + c = 3)

Equation 3 - Equation 2: 5a + b = 2

(5a + b = 2) - (3a + b = 1)

2a = 1

a = 1/2

e. Substitute the value of a = 1/2 in any of the last two equations.

3a + b = 1

3 (1/2) + b = 1

b = 1 - 3/2

b = - 1/2

a + b + c = 2

1/2 - 1/2 + c = 2

c = 2

f. Substitute the values a = 1/2, b = -1/2, and c = 2 in the general equation.

an² + b(n) + c = a_n

(1/2)n² - (1/2)(n) + 2 = a_n

g. Check the general term by substituting the values into the equation.

(1/2)n² - (1/2)(n) + 2 = a_n

a_{n =} 1/2 (n² - n + 4)

a₁ = 1/2 (1² - 1 + 4) = 2

a₂ = 1/2 (2² - 2 + 4) = 3

a₃ = 1/2 (3² - 3 + 4) = 5

a₄ = 1/2 (4² - 4 + 4) = 8

a₅ = 1/2 (5² - 5 + 4) = 12

a₆ = 1/2 (6² - 6 + 4) = 17

a₇ = 1/2 (7² - 7 + 4) = 23

Therefore, the general term of the sequence is:

a_{n =} 1/2 (n² - n + 4)

Problem 3: General Term of Arithmetic Sequence Using Condition 2

Find the general term for the sequence 2, 4, 8, 14, 22, . . .

Solution

a. Create a table of a_n and n values.

n	an
1	2
2	4
3	8
4	14
5	22

b. Take the first and second difference of a_n.

First and Second Difference of the Arithmetic Sequence | Source

c. The second difference is 2. Since the second difference is a constant, therefore the general term of the given sequence is quadratic. Pick three sets of values from the table and form three equations.

General Equation:

an² + b(n) + c = a_n

Equation 1:

at n = 1, a₁ = 2

a (1) + b (1) + c = 2

a + b + c = 2

Equation 2:

at n = 2, a₂ = 4

a (2)² + b (2) + c = 4

4a + 2b + c = 4

Equation 3:

at n = 3, a₂ = 8

a (3)² + b (3) + c = 8

9a + 3b + c = 8

d. Subtract the three equations.

Equation 2 - Equation 1: (4a + 2b + c = 4) - (a + b + c = 2)

Equation 2 - Equation 1: 3a + b = 2

Equation 3 - Equation 2: (9a + 3b + c = 8) - (4a + 2b + c = 4)

Equation 3 - Equation 2: 5a + b = 4

(5a + b = 4) - (3a + b = 2)

2a = 2

a = 1

e. Substitute the value of a = 1 in any of the last two equations.

3a + b = 2

3 (1) + b = 2

b = 2 - 3

b = - 1

a + b + c = 2

1 - 1 + c = 2

c = 2

f. Substitute the values a = 1, b = -1, and c = 2 in the general equation.

an² + b(n) + c = a_n

(1)n² - (1)(n) + 2 = a_n

n² - n + 2 = a_n

g. Check the general term by substituting the values into the equation.

n² - n + 2 = a_n

a_{1 =} 1² - 1 + 2 = 2

a_{2 =} 2² - 2 + 2 = 4

a_{3 =} 3² - 3 + 2 = 8

a_{4 =} 4² - 4 + 2 = 14

a_{5 =} 5² - 5 + 2 = 22

Therefore, the general term of the sequence is:

a_{n =} n² - n + 2

Self-Assessment

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Did you learn from the examples?

Questions & Answers

Question:
How to find general term of sequence 0, 3, 8, 15, 24?
Answer:
The general term for the sequence is an = a(n-1) + 2(n+1) + 1
Helpful 79
Question:
How do I get the formula if the common difference falls on the third row?
Answer:
If the constant difference falls on the third, the equation is a cubic. Try solving it following the pattern for quadratic equations. If it's not applicable, you can solve it using logic and some trial and error.
Helpful 31
Question:
How to find general term of the sequence 4, 12, 26, 72, 104, 142, 186?
Answer:
The general term of the sequence is an = 3n^2 − n + 2. The sequence is quadratic with second difference 6. The general term has the form an = αn^2+βn+γ.To find α, β, γ plug in values for n = 1, 2, 3:
4 = α + β + γ
12 = 4α + 2β + γ
26 = 9α + 3β + γ
and solve, yielding α = 3, β = −1, γ = 2
Helpful 24
Question:
whats is the general term of the set {1,4,9,16,25}?
Answer:
The general term of the sequence {1,4,9,16,25} is n^2.
Helpful 44
Question:
How to find the general term of the sequence 8,13,18,23,...?
Answer:
First thing to do is try to find a common difference.
13 - 8 = 5
18 - 13 = 5
23 - 18 = 5
Therefore the common difference is 5. The sequence is done by adding 5 to the previous term. Recall that the formula for the arithmetic progression is an = a1 + (n - 1)d. Given a1 = 8 and d = 5, substitute the values to the general formula.
an = a1 + (n - 1)d
an = 8 + (n - 1) (5)
an = 8 + 5n - 5
an = 3 + 5n
Therefore, the general term of the arithmetic sequence is an = 3 + 5n
Helpful

Question:
Is there a faster way to calculate the general term of a sequence?
Answer:
Unfortunately, this is the easiest method in finding the general term of basic sequences. You can refer to your textbooks or wait until I get to write another article regarding your concern.
Helpful 15
Question:
What will be nth term of the sequence 4, 12, 28, 46, 72, 104, 142 ...?
Answer:
Unfortunately, this sequence does not exist. But if you replace 28 with 26. The general term of the sequence would be an = 3n^2 − n + 2
Helpful 6
Question:
What is the explicit formula for the nth term of the sequence 1,0,1,0?
Answer:
The explicit formula for the nth term of the sequence 1,0,1,0 is an = 1/2 + 1/2 (−1)^n, wherein the index starts at 0.
Helpful 5
Question:
What is the set builder notation of an empty set?
Answer:
The notation for an empty set is "Ø."
Helpful 4
Question:
What is the general formula of the sequence 3,6,12, 24..?
Answer:
The general term of the given sequence is an = 3^r^(n-1).
Helpful 2
Question:
How do i find the general term of a sequence 9,9,7,3?
Answer:
If this is the correct sequence, the only pattern I see is when you start with number 9.
9
9 - 0 = 9
9 - 2 = 7
9 - 6 = 3
Therefore.. 9 - (n(n-1)) where n starts with 1.
If not, I believe there is a mistake with the sequence you provided. Please try to recheck it.
Helpful 1
Question:
Is there another way of finding general term of sequences using condition 2?
Answer:
There are a lot of ways in solving the general term of sequences, one is trial and error. The basic thing to do is writing down their commonalities and derive equations from those.
Helpful 1
Question:
What is the general form of the sequence 5,9,13,17,21,25,29,33?
Answer:
The general term of the sequence is 4n + 1.
Helpful 1
Question:
What is the general term of the sequence {7,3,-1,-5}?
Answer:
The pattern for the given sequence is:
7
7 - 4 = 3
3 - 4 = -1
-1 - 4 = -5
All succeeding terms are subtracted by 4.
Helpful
Question:
How to find general pattern of 3/7, 5/10, 7/13,...?
Answer:
For fractions, you can separately analyze the pattern in the numerator and the denominator.
For the numerator, we can see that the pattern is by adding 2.
3
3 + 2 = 5
5 + 2 = 7
or by adding multiples of 2
3
3 + 2 = 5
3 + 4 = 7
Therefore the general term for the numerator is 2n + 1.
For the denominator, we can observe that the pattern is by adding 3.
7
7 + 3 = 10
10 + 3 = 13
Or by adding multiples of 3
7
7 + 3 = 10
7 + 6 = 13
Therefore, the pattern for the denominator is 3n + 4.
Combine the two patterns and you'll come up with (2n + 1) / (3n + 4) which is the final answer.
Helpful
Question:
How to find general term for sequence given as an=3+4a(n-1) given a1=4?
Answer:
So you mean how to find the sequence given the general term. Given the general term, just start substituting the value of a1 in the equation and let n =1. Do this for a2 where n=2 and so on and so forth.
Helpful
Question:
What if there is no common difference for all the rows?
Answer:
if there is no common difference for all the rows, try to identify the flow of the sequence through trial and error method. You must identify the pattern first before concluding an equation.
Helpful
Question:
General term for the sequence {1,4,13,40,121} ?
Answer:
1
1+3 = 4
1+3+3^2 = 13
1+3+3^2+3^3 = 40
1+3+3^2+3^3+3^4 = 121
So, the general term of the sequence is a(sub)n=a(sub)n-1 + 3^(n-1)
Helpful

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Comments

AUTHOR
Ray
2 weeks ago from Philippines
For those who want to learn finding the nth term of arithmetic sequences and geometric sequences, please leave your comments here in the comment section. Thank you!
AUTHOR
Ray
2 months ago from Philippines
Technically, the sequences you provided are not basic sequences and requires an advanced knowledge in the field of mathematics. For instance, the general term of the sequence 1,3,11,43… is n! multiplied by the summation of 1/(n-k) where k starts with 0. Soon, I’ll create an article discussing about these type of sequences.
Akinwunmi Sulaiman Awwal
3 months ago
how to get general term for the sequence
3, 10, 22, 35, 49,
2, 10, 25, 54,
2, 5, 11, 23, 38,
1, 3, 11, 43,
1, 2, 5, 15, 54,
1, 4, 13, 47, 185,
Bitaniya bahiru
5 months ago
Thank you so much. it's very helpful
syed ashar ali
6 months ago
I want to find a generalized term of a sequence whose common ratio is an arithmetic sequence
Nasibud Din
7 months ago
I am not understand dicemal and binary system
godfrey sebabatso mokatile
7 months ago
the maths has no problem is need to practice it
Brian.sitton@ciy.com
9 months ago
I think in the first picture the -1 should be a +1.
(in the +7 sequence)
Jonh paul Sallaya
9 months ago
General term
An.sqaure. + bn + c = An
A=1 ,b=1, c= -1
Then whats next pls?
Han
12 months ago
7,13,25,49……
7=6+1=6x2^0+1
13=6x2+1=6x2^1+1
25=6x4+1=6x2^2+1
49=6x8+1=6x2^3+1……So:
an=6x2^(n-1)+1
Cody
15 months ago
Then what is the sequence for 7,13,25,49.........
lol
17 months ago
this really helped me, thanks
Rowan
17 months ago
That arithmetic sequence is super wrong. -6 + 7 is not -1
Srijan Guha
19 months ago
Easily understandable and very helpful mathematical article.
Nicole Ann Cuevas
20 months ago
Thanks! Its a big help!
AUTHOR
Ray
21 months ago from Philippines
Thank you, Ma'am Chitrangada Sharan. I really like to help students having trouble with their Mathematics subject. Have a great day!
Chitrangada Sharan
21 months ago from New Delhi, India
Your article is educative and informative! Very well explained and useful for many.
Thanks for sharing this !

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How to Find the General Term of Sequences

What Is a Sequence?

What Is an Arithmetic Sequence?

Steps in Finding the General Formula of Arithmetic and Geometric Sequences

Problem 1: General Term of an Arithmetic Sequence Using Condition 1

Solution

Problem 2: General Term of Arithmetic Sequence Using Condition 2

Solution

Problem 3: General Term of Arithmetic Sequence Using Condition 2

Solution

Self-Assessment

Did you learn from the examples?

Questions & Answers

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How to Learn Algebra Fast—Rules, Equations, Solutions

Parabola Equations and Graphs, Directrix and Focus and How to Find Roots of Quadratic Equations

How to Create Your Own Simple Linear Regression Equation

How to Graph a Circle Given a General or Standard Equation

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How to Find the General Term of Sequences

What Is a Sequence?

What Is an Arithmetic Sequence?

Steps in Finding the General Formula of Arithmetic and Geometric Sequences

Problem 1: General Term of an Arithmetic Sequence Using Condition 1

Solution

Problem 2: General Term of Arithmetic Sequence Using Condition 2

Solution

Problem 3: General Term of Arithmetic Sequence Using Condition 2

Solution

Self-Assessment

Did you learn from the examples?

Questions & Answers

Related

Popular

Comments

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