IXL - Find a recursive formula (Precalculus practice)

Which recursive formula can be used to define this sequence for

–

1, 6, 11, 16, 21, ...

–

ref_doc_title.

Learn with an example

question

Which recursive formula can be used to define this sequence for

9, 12, 15, 18, 21, 24, ...

–

key idea

A sequence is an ordered list of numbers, called

terms.

The terms in a sequence are sometimes written

etc., where

is the first term,

is the second term, and so on. The subscript

on the term

is called the

index.

A recursive formula defines a sequence using earlier terms.

solution

To find the recursive formula, start by looking at the differences and ratios of consecutive terms. In this case, the differences between consecutive terms are all the same.

n	a n	a n – a n – 1
1	9	undefined
2	12	3
3	15	3
4	18	3
5	21	3
6	24	3

The table shows that

–

for

Since the difference between consecutive terms is always the same, this sequence is

arithmetic.

Write this rule as a recursive formula.

–

Since the first term in the sequence is

the term

–

is not defined for

≤

So, this formula is only valid for

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Teacher tools

Work it out

Not feeling ready yet? These can help:

MM.5Classify formulas and sequences

AA.2Find terms of a recursive sequence

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