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Quadratic Sequences: The Nth Term of a Quadratic Number Sequence

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Mark, a math enthusiast, loves writing tutorials for stumped students and those who need to brush up on their math skills.

Read on to learn a useful mathematical calculation.

Read on to learn a useful mathematical calculation.

Step-by-Step

Here, we will be finding the nth term of a quadratic number sequence. A quadratic number sequence has nth term = an² + bn + c

Example 1

Write down the nth term of this quadratic number sequence.

-3, 8, 23, 42, 65...

Step 1: Confirm the sequence is quadratic. This is done by finding the second difference.

Sequence = -3, 8, 23, 42, 65

1st difference = 11,15,19,23

2nd difference = 4,4,4,4

Step 2: If you divide the second difference by 2, you will get the value of a.

4 ÷ 2 = 2

So the first term of the nth term is 2n²

Step 3: Next, substitute the number 1 to 5 into 2n².

n = 1,2,3,4,5

2n² = 2,8,18,32,50

Step 4: Now, take these values (2n²) from the numbers in the original number sequence and work out the nth term of these numbers that form a linear sequence.

n = 1,2,3,4,5

2n² = 2,8,18,32,50

Differences = -5,0,5,10,15

Now the nth term of these differences (-5,0,5,10,15) is 5n -10.

So b = 5 and c = -10.

Step 5: Write down your final answer in the form an² + bn + c.

2n² + 5n -10

Example 2

Write down the nth term of this quadratic number sequence.

9, 28, 57, 96, 145...

Step 1: Confirm if the sequence is quadratic. This is done by finding the second difference.

Sequence = 9, 28, 57, 96, 145...

1st differences = 19,29,39,49

2nd differences = 10,10,10

Step 2: If you divide the second difference by 2, you will get the value of a.

10 ÷ 2 = 5

So the first term of the nth term is 5n²

Step 3: Next, substitute the number 1 to 5 into 5n².

n = 1,2,3,4,5

5n² = 5,20,45,80,125

Step 4: Now, take these values (5n²) from the numbers in the original number sequence and work out the nth term of these numbers that form a linear sequence.

n = 1,2,3,4,5

5n² = 5,20,45,80,125

Differences = 4,8,12,16,20

Now the nth term of these differences (4,8,12,16,20) is 4n. So b = 4 and c = 0.

Step 5: Write down your final answer in the form an² + bn + c.

5n² + 4n

Questions & Answers

Question: Find the nth term of this sequence 2,8,18,32,50?

Answer: The first differences are 6,10,14,18, and the second differences are 4.

Therefore the first term of the sequence is 2n^2.

Subtracting 2n^2 from the sequence gives 0.

So the formula is just 2n^2.

Question: What is the nth term of this quadratic sequence: 4,7,12,19,28?

Answer: The first differences are 3, 5, 7, 9 and the second differences are 2.

Hence, the first term of the sequence is n^2 (since half of 2 is 1).

Subtracting n^2 from the sequence gives 3, 3, 3, 3, 3.

So putting these two terms together gives n^2 + 3.

Question: Find the nth term of this quadratic sequence 3,11,25,45?

Answer: The first differences are 8, 14, 20.

The second differences are 6.

Half of 6 is 3, so the first term of the sequence is 3n^2.

If you subtract 3n^2 from the sequence you get 0,-1,-2,-3 which has the nth term of -n + 1.

Therefore your final answer will be 3n^2 - n + 1

Question: Find the nth term of 3,8,15,24?

Answer: The first differences are 5, 7, 9, and the second differences are all 2, so the sequence must be quadratic.

Half of 2 gives 1, so the first term of the nth term is n^2.

Subtracting n^2 from the sequence gives 2, 4, 6, 8 which has nth term of 2n.

So putting both terms together gives n^2 + 2n.

Question: Can you find nth term of this quadratic sequence 2,8,18,32,50?

Answer: This is just the square number sequence doubles.

So if the square numbers have nth term of n^2, then the nth term of this sequence is 2n^2.

Question: Find the nth term of this sequence 6, 12, 20, 30, 42, 56, 72?

Answer: First differences are 6, 8, 10, 12, 14, 16.

Second differences are 2.

First term is therefore n^2 (Since half of 2 is 1)

Subtracing n^2 from the sequence gives 5, 8, 11, 14, 17, 20, 23 which has nth term 3n + 2.

So the final answer is n^2 + 3n + 2.

Question: What is the ninth term of this sequence 6,12,20,30,42,56?

Answer: The first differences are 6,8,10,12,14. The second difference is 2. Therefore half of 2 is 1 so the first term is n^2. Subtract this from the sequence gives 5,8,11,14,17. The nth term of this sequence is 3n + 2. So the final formula for this sequence is n^2 + 3n + 2.

Question: Find the first three terms of this 3n+2?

Answer: You can find the terms by substituting 1,2 and 3 into this formula.

This gives 5,8,11.

Question: What is the nth term of this sequence:12, 17, 24, 33, 44, 57, 72?

Answer: The first differences are 5,7,9,11,13,15, and the second differences are 2.

This means that the first term of the sequence is n^2.

Subtracting n^2 from the sequence gives 11,13,15,17,19,21, which has nth term of 2n + 9.

So putting these together gives a nth term of the quadratic sequence of n^2 + 2n + 9.

Question: Find the nth term of this sequence 4,13,28,49,76?

Answer: The first differences of this sequence are 9, 15, 21, 27, and the second differences are 6.

Since half of 6 is 3 then the first term of the quadratic sequence is 3n^2.

Subtracting 3n^2 from the sequence gives 1 for each term.

So the final nth term is 3n^2 + 1.

Question: What is the nth term of 3,8,17,30,47?

Answer: The first differences are 5, 9, 13, 17, and so the second differences are all 4.

Halving 4 gives 2, so the first term of the sequence is 2n^2.

Subtracting 2n^2 from the sequences gives 1,0,-1-2,-3 which has the nth term -n+2.

Therefore, the formula for this sequence is 2n^2 -n +2.

Question: What is the nth term of 6, 9, 14, 21, 30, 41?

Answer: These numbers are 5 more than the square number sequence 1,4,9,16,25,36 which has nth term n^2.

So the final answer for the nth term of this quadratic sequence is n^2 + 5.

Question: Find nth term of this sequence 4,11,22,37?

Answer: The first differences are 7, 11, 15, and the second differences are 4.

Since half of 4 is 2, then the first term will be 2n^2.

Subtracting 2n^2 from the sequence gives 2, 3, 4, 5 which has nth term n + 1.

Therefore the final answer is 2n^2 + n + 1.

Question: Can you find the nth term of this sequence 8, 14, 22, 32, 44, 58, 74?

Answer: The first differences are 6,8,10,12,14,16 and the second differences are 2.

Therefore the first term in the quadratic sequence is n^2.

Subtracting n^2 from the sequence gives 7, 10, 13, 15, 18, 21, and the nth term of this linear sequence is 3n + 4.

So the final answer of this sequence is n^2 + 3n + 4.

Question: Find the nth term of this sequence 7,10,15,22,31?

Answer: These numbers are 6 more than the square numbers, so the nth term is n^2 + 6.

Question: What is the Nth term of 2, 6, 12, 20?

Answer: The first differences are 4, 6, 8, and the second differences are 2.

This means the first term is n^2.

Subtracting n^2 from this sequence gives 1, 2, 3, 4 which has nth term n.

So the final answer is n^2 + n.

Question: Find the nth term for 7,9,13,19,27 ?

Answer: The first differences are 2, 4, 6, 8, and the second differences are 2.

Since half of 2 is 1, then the first term of the sequence is n^2.

Subtracting n^2 from the sequence gives 6,5,4,3,2 which has nth term -n + 7.

So the final answer is n^2 - n + 7.

Question: Find nth term of this sequence 10,33,64,103?

Answer: The first differences are 23, 31, 39 and the second difference is 8.

Therefore since half of 8 is 4 the first term will be 4n^2.

Subtracting 4n^2 from the sequence gives 6, 17, 28 which has nth term 11n - 5.

So the final answer is 4n^2 + 11n -5.

Question: Find the nth term of this quadratic sequence? 4,7,12,19,28

Answer: These numbers are 3 more than the square number sequence so the nth term will ne n^2 + 3.

Question: What is the sequence formula for 6,17,34,57,86?

Answer: The first differences are 11, 17, 23, 29.

The second differences are 6.

Half of 6 is 3, so the first term of the sequence is 3n^2.

If you subtract 3n^2 from the sequence you get 3,5,7,9 which has the nth term of 2n + 1.

Therefore your final answer will be 3n^2 + 2n + 1.

Question: what is the nth term of:4,9,16,25,36 ?

Answer: The first differences are 5, 7, 9, 11.

The second differences are 2.

Half of 2 is 1, so the first term of the sequence is n^2.

If you subtract n^2 from the sequence you get 3,5,7,9,11 which has the nth term of 2n + 1.

Therefore your final answer will be n^2 + 2n + 1.

Question: Find the nth term of this sequence -5,-2,3,10,19?

Answer: These numbers are 6 smaller than the square number sequence (1,4,9,16,25...)

Therefore the nth term is n^2 - 6.

Question: Can you find the nth term of this sequence 8,16,26,38,52?

Answer: The first difference of the sequence are 8, 10, 12, 24.

The second differences of the sequences are 2, therefore since half of 2 is 1 then the first term of the sequence is n^2.

Subtracting n^2 from the given sequence gives, 7,12,17,22,27. The nth term of this linear sequence is 5n + 2.

So if you put the three-term together, this quadratic sequence has the nth term n^2 + 5n + 2.

Question: Find the nth term of the sequence 302,600,894,1184?

Answer: The first differences are 298, 294, 290.

The second differences are -4.

Half of -4 is 2, so the first term of the sequence is -2n^2.

If you subtract n^2 from the sequence gives 304, 608, 912, 1216 which has nth term 304n.

Therefore your final answer will be -2n^2 + 304n.

Question: What are the first 4 term of n squared +3?

Answer: Sub in n=1, 1^2 + 3 = 4.

Next sub in n=2, 2^2 + 3 = 7.

Next sub in n = 3, 3^2 + 3 = 12.

Next sub in n = 4, 4^2 + 3 = 19.

So the first 4 terms are 4,7,12,19.

Question: Find the sequence from 2n^2-1?

Answer: First start with n^2 which is 1,4,9,16,25...

Now multiply these numbers by 2 to give 2,8,18,32,50...

Finally subtract 1 from these numbers to give 1,7,17,31,49...

Question: Can you find the nth term of 3, 10, 21, 36, 55?

Answer: First differences are 7, 11, 15, 19.

Second differences are 4.

The first term is therefore 2n^2 (Since half of 4 is 2).

Subtracting 2n^2 from the sequence gives 1, 2, 3, 4, 5, which has nth term n.

So the final answer is 2n^2 + n.

Question: What is the nth term of this sequence 3,8,15,24,35?

Answer: The first differences are 5, 7, 9, 11, and the second differences are 2.

This means the first term is n^2 (since half of 2 is 1).

Subtracting n^2 from the sequence gives 2, 4, 6, 8, 10 which has nth term 2n.

So the final answer is n^2 + 2n.

Question: Find the nth term of this quadratic sequence 6,12,20,30,42,56,72?

Answer: The first differences are 6,8,10,12,14,16, and the second differences are 2.

Therefore the first term is n^2 (as half of 2 is 1).

Subtracting n^2 from the sequence gives 5, 8, 11,14,17,20,23 which has nth term of 3n + 2.

So the final nth term is n^2 + 3n + 2.

Question: Find the expression of this sequence? -5,-2,5,16,31

Answer: The first differences are 3,7,11,15, and the second differences are 4.

Therefore the first term of the sequence is 2n^2.

Subtracting 2n^2 from the sequence gives -7, -10, -13, -16, -19 which has nth term -3n - 4.

So the formula is 2n^2 -3n - 4.

Question: Find the nth term of this sequence 9,23,43,69,101,139?

Answer: The first differences are 14, 20, 26, 32, 38 and the second differences are 6, meaning that the first term is 3n^2.

Subtracting 3n^2 from the sequence gives 6, 11, 16, 21, 26, 31 which has nth term 5n + 1.

So the final answer is 3n^2 + 5n + 1.

Question: What is the nth term formula for the sequence which begins 87, 74, 61, 48?

Answer: This sequence is linear and not quadratic.

The first differences are -13, so compare the sequence with -13, -26,- 39, -52.

This will give a final answer of -13n + 100.

Question: What is the nth term of the sequence 0,3,8,15,24?

Answer: These numbers are one smaller than the square number sequence, so it will be n^2 - 1.

Question: Find the nth term of this sequence -2,1,6,13,22,33?

Answer: The numbers in this quadratic sequence are 3 smaller than the square number sequence, so the nth term is n^2 - 3.

Question: Find the nth term of this sequence 4,7,12,19,28?

Answer: First, work out the first differences; these are 3, 5, 7, 9.

Next, find the second differences, these are all 2.

So since half of 2 is 1, then the first term is n^2.

Subtracting n^2 from the sequence gives 3.

So the nth term of this quadratic sequence is n^2 + 3.

Question: Can you find the nth term of this sequence 1, 3, 7, 13, 21?

Answer: The first differences are 2,4,6,8, and the second differences are 2.

Therefore, the first term will be n^2.

Subtracting n^2 from the sequence gives 0,-1,-2,-3,-4 which has nth term of -n + 1.

So the final answer is n^2 - n + 1.

Question: What is the nth term rule of the quadratic sequence below? 7,14,23,34,47,62,79

Answer: The first differences are 7,9,11,13,15,17,19,21 and the second difference are 2.

Half of 2 is 1, so the first term is n^2.

Subtract n^2 from the sequence gives 6, 10, 14, 18, 22, 26, 30 which has nth term 4n + 2.

So the nth term of the sequence is n^2 + 4n + 2.

Question: Find the nth term of the sequence 1,7,17,31?

Answer: The first differences are 6, 10, 14.

The second differences are 4.

Half of 4 is 2, so the first term of the sequence is 2n^2.

If you subtract 2n^2 from the sequence you get -1 for each term.

Therefore your final answer will be 2n^2 - 1

Question: What is the Nth term of 4,9,16,25,36 ?

Answer: These are the square numbers, excluding the first term of 1.

Therefore, the sequence has a Nth term of (n+1)^2.

Question: Find the nth term of this sequence 7, 14, 23, 34, 47, 62, 79 ?

Answer: The first differences are 7,9,11,13,15,17 and the second differences are 2.

This means that the first term of the sequence is n^2.

Subtracting n^2 from the sequence gives 6,10,14,18,22,26, which has nth term of 4n + 2.

So putting these together gives a nth term of the quadratic sequence of n^2 + 4n + 2.

Question: Find the nth term of this sequence 2,9,20,35,54?

Answer: The first differences are 7, 11, 15, 19.

The second differences are 4.

Half of 4 is 2, so the first term of the sequence is 2n^2.

If you subtract 2n^2 from the sequence you get 0,1,2,3,4 which has the nth term of n - 1

Therefore your final answer will be 2n^2 + n - 1

Question: Find the nth term of this sequence 3,8,15,24,35?

Answer: The first differences are 5, 7, 9, 11, and so the second differences are all 2.

Halving 2 gives 1, so the first term of the sequence is n^2.

Subtracting n^2 from the sequences gives 2,4,6,8,10 which has the nth term 2n.

Therefore, the formula for this sequence is n^2 + 2n.

Question: What is the nth term of this quadratic number sequence: 1,3,6,10,15?

Answer: The first differences are 2,3,4,5 and the second differences are 1.

Therefore the first term in the quadratic sequence is 0.5n^2.

Subtracting 0.5n^2 from the sequence gives 0.5, 1, 1.5, 2, 2.5, and the nth term of this linear sequence is 0.5n.

So the final answer of this sequence is 0.5n^2 + 0.5n.

Question: Find the nth term of this sequence 4,9,16,25,36?

Answer: These numbers are 1 ahead in the square number sequence.

So the nth term is (n+1)^2.

Question: What are the first three terms for n^(2)+20?

Answer: To work this out the numbers 1,2 and 3 need to be substituted into the nth term.

So the first term is 1^2 + 20 = 21.

The second term is 2^2 + 20 = 24.

The third term is 3^2 + 20 = 29.

So the first three terms are 21,24,29.

Question: What is the nth term of this quadratic number sequence 5, 9, 1, 29?

Answer: The first differences are 4, 8, 12, and the second differences are 4.

Since half of 4 is 2 then the first term of the nth term is 2n^2.

Subtracting 2n^2 from this sequence gives 3, 1, -1, -3 which has nth term of -2n + 5.

So putting this together gives 2n^2 -2n + 5.

Question: Can you find the nth term of 11, 26, 45 and 68?

Answer: The first differences are 15, 19 and 23. The second differences are 4.

Half of 4 is 2, so the first term is 2n^2.

Subtracting 2n^2 from the sequence gives you 9, 18, 27 and 36, which has nth term 9n.

So, the final formula for this quadratic sequence is 2n^2 + 9n.

Question: What is the nth term of this: 3,18,41,72,111?

Answer: The first differences are 15,23,31,39, and the second differences are 8.

Halving 8 gives 4, so the first term of the formula is 4n^2

Now subtract 4n^2 from this sequence to give -1,2,5,8,11, and the nth term of this sequence is 3n – 4.

So the nth term of the quadratic sequence is 4n^2 + 3n – 4.

Question: What is the nth term rule of this quadratic sequence: 8, 14, 22, 32, 44, 58, 74?

Answer: The first differences are 6, 8, 10, 12, 14, 16, and so the second differences are all 2.

Halving 2 gives 1, so the first term of the sequence is n^2.

Subtracting n^2 from the sequences gives 7,10,13,16,19,22 which has the nth term 3n + 4.

Therefore, the formula for this sequence is n^2 + 3n + 4.

Question: What is the nth term of 6, 20, 40, 66, 98,136?

Answer: The first differences are 14, 20, 26, 32 and 38, and so the second differences are all 6.

Halving 6 gives 3, so the first term of the sequence is 3n^2.

Subtracting 3n^2 from the sequences gives 3,8,13,18,23 which has the nth term 5n-2.

Therefore, the formula for this sequence is 3n^2 + 5n - 2.

Comments

hellom8 on February 14, 2019:

this is very helpful , cheers

confusedchild on September 05, 2018:

im still a bit confused how you work out 'c' like how did you get -10 / 0?

louisearabiana on June 18, 2018:

where does this difference came from, like the 4,8,12,16,20?

Mark (author) from England, UK on November 24, 2017:

This would rarely happen, but you can still apply the same method. Just put a decimal before n squared.

its ya boi on November 19, 2017:

What if the second difference is an odd number?

Silver49 on January 05, 2014:

Very good explanation I will recommend this site to any one who has got problem understanding quadratic sequence

MKING on September 12, 2013:

its it the same if you get for example 3n+3n+3n+1 like 3 nth terms do u just go and make a third sequence from your second sequence

cool guy on June 17, 2013:

what do you do if the number only repeats after 4 differences:

1 1 2 7 21

0 1 5 14 1

1 4 9 2

3 5 3

2 4

What is the equation?

Gervasius Stephanus on May 09, 2013:

Conceptual undertanding is very important for students to comprehend quadratic sequence. Hence, the logic of determining the terms of a given sequence defined by a quadratic formula should be the starting point.

Dr Gervasius Stephen on May 09, 2013:

The procedure helps far better but understanding of the concept per se in changing the sequence to arithmetic number sequence should be greatly emphasised.

MathFabPhobiaCured on April 23, 2013:

dat is just dam cul. luving dis site. helpd me a lot.

quad eqns made easy for us math phobs people.

shafster on November 30, 2012:

its epic

sid on May 17, 2012:

brilliant i was having trouble with quadratic sequences and this helped a lot thanks

Angel on February 12, 2012:

N is number of term.

Angel on February 12, 2012:

Dats cul

matthias caruana on November 27, 2011:

In step one there's a mistake in the first difference

it should be 11,15,19,23

Me. on April 04, 2011:

thanks for this, had trouble understanding for my maths homework