What is a quadratic sequence?
A quadratic sequence is one whose first difference varies but whose second difference is constant.
What is the nth term of a quadratic sequence?
The nth term of a quadratic sequence takes the form of: an2 + bn + c
We see why it’s called a quadratic sequence; the nth term has an n2 in it.
a is the 2nd difference divided by 2.
c is the zeroth term.
How do you find the nth term of a quadratic sequence?
Look at the sequence: 3, 9, 19, 33, 51, …
The second difference is 4.
The zeroth term is the term which would go before the first term if we followed the pattern back.
Working backwards, we know the second difference will be 4.
So the first difference between the terms in position 0 and 1 will be 6 − 4 = 2.
And the zeroth term will be 3 − 2 = 1.
In the sequence: 3, 9, 19, 33, 51, …
we calculated the zeroth term as 1 and the 2nd difference as 4.
Remember, nth term = an2 + bn + c
where a is the 2nd difference ÷ 2 and c is the zeroth term
In this example, a = 2 and c = 1
We now have: nth term = 2n2 + bn + 1
We need to find the value of b.
So far… in the sequence: 3, 9, 19, 33, 51, …
we know that the nth term = 2n2 + bn + 1
The 4th term in the sequence is 33. So, substituting that into the formula for the nth term will help us to find the value of b:
2 × 42 + 4 × b + 1 = 33
32 + 4b + 1 = 33
4b = 0 and b = 0
Now that we have found the value of b, we know the nth term = 2n2 + 1
Try checking it by working out, for example, the 3rd term and checking it with the sequence.
Seconday resources for finding the nth term of a quadratic sequence
These KS3 maths resources are great for practising and applying finding the nth term of a quadratic sequence:
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