Recursive Formula - Rule of Arithmetic and Geometric Sequence (2024)

Before going to learn the recursive formula, let us recall what is a recursive function. A recursive function is a function that defines each term of a sequence using a previous term that is known, i.e. where the next term is dependent on one or more known previous term(s). A recursive function h(x) can be written as:

h(x) = a0 h(0) + a1h(1) + ....... + ax-1 h(x-1) where ai ≥ 0 and at least one of the ai > 0

Let us learn the recursive formulas in the following section.

What Are Recursive Formulas?

A recursive formula refers to a formula that defines each term of a sequence using the preceding term(s). The recursive formulas define the following parameters:

  • The first term of the sequence
  • The pattern rule to get any term from its previous term

Recursive Formula - Rule of Arithmetic and Geometric Sequence (1)

Recursive Formulas

The following are the recursive formulas for different kinds of sequences.

Recursive Formula for Arithmetic Sequence

The recursive formula to find the nth term of an arithmetic sequence is:

an = an-1 + d for n ≥ 2

where

  • an is the nth term of a A.P.
  • d is the common difference.

Recursive Formula for Geometric Sequence

The recursive formula to find the nth term of a geometric sequence is:

an = an-1 r for n ≥ 2

where

  • an is the nth term of a G.P.
  • r is the common ratio.

Recursive Formula for Fibonacci Sequence

The recursive formula to find the nth term of a Fibonacci sequence is:

an = an-1 + an-2 for n ≥ 2, where

  • a0 = 1 and
  • a1 = 1

where an is the nth term of the sequence.

Let us see the applications of the recursive formulas in the following section.

Recursive Formula - Rule of Arithmetic and Geometric Sequence (2)

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Example 1: The recursive formula of a function is, f(x) = 5 f(x-2) + 3, find the value of f(8). Given that f(0) = 0.

Solution:

f(8) = 5 f(6) + 3
f(6) = 5 f(4) + 3
f(4) = 5 f(2) + 3

It is given that f(0) = 0. Thus,

f(2) = 5 f(0) + 3 = 3
f(4) = 5 × 3 + 3 = 18
f(6) = 5 × 18 + 3 = 93
f(8) = 5 × 93 + 3 = 468

Answer: The value of f(8) is 468.

Example 2: Find the recursive formula for the following arithmetic sequence: 1, 6, 11, 16 .....

Solution:

Let an be the nth term of the series and d be the common difference.

d = a2 - a1 = 6 - 1 = 5

an = an-1 + 5

Answer: The recursive formula for this sequence is an = an-1 + 5

Example 3: The 13th and 14th terms of the Fibonacci sequence are 144 and 233 respectively. Find the 15th term.

Solution:

Using the recursive formula for the Fibonacci sequence,

15th term is the sum of 13th term and 14th term.

15th term = 13th term + 14th term

= 144 + 233

= 377

Answer: The 15th term of the Fibonacci sequence is 377.

FAQs on Recursive Formula

What is the Recursive Formula in Math?

A recursive formula is a formula that defines any term of a sequence in terms of its preceding term(s). For example:

  • The recursive formula of an arithmetic sequence is, an = an-1 + d
  • The recursive formula of a geometric sequence is, an = an-1r

Here, an represents the nth term and an-1 represents the (n-1)th term.

How to Find the Recursive Formula for an Arithmetic Sequence?

To find a recursive sequence in which terms are defined using one or more previous terms which are given.

  • Step 1: Identify the nth term (an) of an arithmetic sequence and the common difference, d,
  • Step 2: Put the values in the formula, an+1 = an + d to find the (n+1)th term to find the successive terms.

What is the Recursive Rule For the Fibonacci series?

The Fibonacci series is characterized as the series in which each number is the sum of two numbers preceding it in the sequence. Thus, the Fibonacci formula is given as, Fn = F(n-1) + F(n-2), where n > 1.

What Is an in Recursive Formula?

In any recursive formula, an refers to the nth term in the sequence, which can be found using the recursive formulas:

  • nth term of A.P: an = an-1 + d for n ≥ 2
  • nth term of G.P: an = an-1 for n ≥ 2
  • nth term of a Fibonacci Sequence: an = an-1 + an-2 for n ≥ 2

What is the Difference Between Recursive and Explicit Formulas?

The recursive formula is used to find a term of a sequence when its previous term is known. The explicit formula is used to find the term of a sequence irrespective of information about its previous term. For example:

  • The recursive formula to find the nth term of AP is: an = an-1 + d
  • The explicit formula to find the nth term of AP is, an = a + (n - 1) d
Recursive Formula - Rule of Arithmetic and Geometric Sequence (2024)

FAQs

Recursive Formula - Rule of Arithmetic and Geometric Sequence? ›

The recursive formula of an arithmetic sequence is, an = an-1 + d. The recursive formula of a geometric sequence is, an = an-1r.

How to do a recursive formula for a geometric sequence? ›

For a geometric sequence with recurrence of the form a(n)=ra(n-1) where r is constant, each term is r times the previous term. This implies that to get from the first term to the nth term, we need to multiply by n-1 factors of r.

What is the recursive formula for an arithmetic sequence? ›

i.e., any term (nth term) of an arithmetic sequence is obtained by adding the common difference (d) to its previous term ((n - 1)th term). i.e., the recursive formula of the given arithmetic sequence is, an=an−1+d a n = a n − 1 + d .

What is the recursive relation formula for the arithmetic sequence? ›

The general form of a recursive formula for an arithmetic sequence is { a 1 = c a n = a n − 1 + d , where is the first term of the sequence, is ith term of the sequence, and is the common difference between each consecutive term of the sequence.

How do you solve a recursive rule? ›

First, identify the common difference (how much each term in a sequence is increasing or decreasing from the previous term). State the first term of the sequence, and then write the recursive rule as (new term) = (previous term) + (common difference).

What is the recursive rule for arithmetic and geometric sequences? ›

A recursive formula is a formula that defines any term of a sequence in terms of its preceding term(s). For example: The recursive formula of an arithmetic sequence is, an = an-1 + d. The recursive formula of a geometric sequence is, an = an-1r.

What is the recursive rule for this geometric sequence 7 21 63189? ›

This is a geometric sequence since there is a common ratio between each term. In this case, multiplying the previous term in the sequence by 3 gives the next term. In other words, an=a1rn−1 a n = a 1 r n - 1 .

What is the explicit rule for a geometric sequence? ›

The explicit formula for a geometric sequence is of the form an = a1 [ r-1 ] , where r is the common ratio.

What is the common recursive formula? ›

There are few recursive formulas to find the nth term based on the pattern of the given data. They are, nth term of Arithmetic Progression an = an 1 + d for n ≥ 2. nth term of Geometric Progression an = an 1 × r for n ≥ 2.

How do you find the formula for an arithmetic sequence? ›

Answer: The formula for the nth term in an arithmetic sequence is an=a1+(n−1)d. This formula can be used to determine the value of any term in an arithmetic sequence. An arithmetic sequence has a common difference between every term.

What is the difference between arithmetic and geometric sequences? ›

Arithmetic sequences are defined by an initial value and a common difference, with the same number added or subtracted to each term. Geometric sequences are defined by an initial value and a common ratio, with the same number multiplied or divided to each term.

How to do recursive? ›

Basic steps of recursive programs
  1. Initialize the algorithm. ...
  2. Check to see whether the current value(s) being processed match the base case. ...
  3. Redefine the answer in terms of a smaller or simpler sub-problem or sub-problems.
  4. Run the algorithm on the sub-problem.
  5. Combine the results in the formulation of the answer.
Apr 27, 2017

How do you write a recursive formula for the nth term? ›

Recursive: nth term=(n-1) term+d. Geometric: First term: a. Ratio: r. Explicit: nth term=a(r)^(n-1).

How to write a recursive formula for a geometric sequence? ›

A recursive formula for a geometric sequence with common ratio r is given by an=ran–1 for n≥2. As with any recursive formula, the initial term of the sequence must be given. See Example 11.3. 3.

Which is the recursive formula for this arithmetic sequence? ›

The recursive equation for an arithmetic squence is: f(1) = the value for the 1st term. f(n) = f(n-1) + common difference. Hope this helps.

What is the geometric formula? ›

What is the rule for the geometric sequence? Each term of a geometric sequence is formed by multiplying the previous term by a constant number r, starting from the first term a1. Therefore, the rule for the terms of a geometric sequence is an=a1(r)^(n-1).

What is the recursive formula for the geometric distribution? ›

There are two geometric probability formulas: Geometric distribution PMF: P(X = x) = (1 - p)x - 1p. Geometric distribution CDF: P(X ≤ x) = 1 - (1 - p)

How do you find the recursive of a sequence? ›

A recursive sequence is a sequence in which terms are defined using one or more previous terms which are given. If you know the term of an arithmetic sequence and you know the common difference , , you can find the ( n + 1 ) th term using the recursive formula a n + 1 = a n + d .

What is the formula for the geometric sequence? ›

Each term of a geometric sequence is formed by multiplying the previous term by a constant number r, starting from the first term a1. Therefore, the rule for the terms of a geometric sequence is an=a1(r)^(n-1).

What is the recursive formula for the geometric sequence 2 10 50 250? ›

Expert-Verified Answer

The recursive formula for the geometric sequence 2, -10, 50, -250, ... is an = an-1 × -5, given that the first term is 2.

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