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### Course: Algebra 1 > Unit 9

Lesson 6: General sequences# Sequences: FAQ

Frequently asked questions about sequences

## What is the difference between an arithmetic sequence and a geometric sequence?

In an arithmetic sequence, we add the same number to each term in order to get the next term in the sequence. In a geometric sequence, we multiply each term by the same number in order to get the next term.

## How do we construct arithmetic sequences?

Start with the first term of the sequence, which can be any number. Then, choose a common difference. This is the number we will add to each term in order to get the next term. For example, if we start with $5$ and have a common difference of $3$ , our sequence will be $5,8,11,14,17,20\dots $

Practice with our Extend arithmetic sequences exercise.

## How do we construct geometric sequences?

Similar to arithmetic sequences, we start with the first term of the sequence. Then, we choose a common ratio. This is the number we will multiply each term by in order to get the next term. For example, if we start with $2$ and have a common ratio of $3$ , our sequence will be $2,6,18,54,162,486\dots $

Practice with our Extend geometric sequences exercise.

## What is the difference between a recursive formula and an explicit formula for a sequence?

An explicit formula allows us to calculate the value of any term in the sequence by plugging in the term number, while a recursive formula defines each term in the sequence in relation to the terms that came before it.

Practice with our Converting recursive & explicit forms of arithmetic sequences exercise.

Practice with our Use geometric sequence formulas exercise.

## What is an example of a recursive formula?

A common example of a recursive formula is the formula for the Fibonacci sequence. The Fibonacci sequence starts with the two terms $0$ and $1$ , and each subsequent term is found by adding the two most recent terms together:

So in this case, the recursive formula would be ${F}_{n}={F}_{n-1}+{F}_{n-2}$ .

Practice with our Recursive formulas for geometric sequences exercise.

## What is an example of an explicit formula?

An example of an explicit formula is the formula for the arithmetic sequence. For any arithmetic sequence, we can find the value of any term by using the formula ${a}_{n}={a}_{1}+(n-1)d$ , where ${a}_{1}$ is the first term in the sequence, $d$ is the common difference, and $n$ is the term number.

Practice with our Explicit formulas for arithmetic sequences exercise.

## Where can we use sequences?

Sequences are important in a variety of real-world applications. For example, financial analysts might use geometric sequences to calculate compound interest or to model the growth of an investment over time. Scientists might use arithmetic sequences to measure the rate at which something is changing. Sequences are also important in mathematics itself, as they can be used to understand patterns and relationships between numbers.

Practice with our Sequences word problems exercise.

## Want to join the conversation?

- Wouldn't it make more sense to call arithmetic sequences "linear sequences"? And maybe geometric sequences are "exponential sequences"? Or is that wrong?(14 votes)
- Correct. Linear sequences mean adding / subtracting the same value from the previous term to get the current term, which is the definition of arithmetic sequence.

Exponential sequences mean multiplying or dividing the same value from the previous term to get the current term, also the definition of geometric sequence.

However if you are asking about the context in this article, the way they assigned Recursive and Explicit to the formulas is correct.(11 votes)

- The notes above say, "An explicit formula allows us to calculate the value of any term in the sequence by plugging in the term number, while a recursive formula defines each term in the sequence in relation to the terms that came before it."

If this is the case, then why in one of the examples does the solution show that the first term in the sequence is 8, the domains in the multiple choice claimed that we can come to that by plugging in 2, 3, and 4. Shouldn't it only be 1 that works out to the first term of the sequence?(9 votes)- I think it depends on the question structure.(2 votes)

- What is a recursive and explicit?please help,and thank you very much
**3**(6 votes)- Recursive formula means you need to compute all required
**previous terms in the sequence**for the formula in order to find the next term.

In the example, Fn = Fn-1 + Fn-2. You simply cannot find (Actually there's a formula but not necessary to mention it now) any term like the F10 term directly. You need to find F9 and F8, which leads to finding F7 and F6 etc.

Explicit formula enables you to just substitute known data and you can get that term in the sequence directly without calculating the terms before it.(8 votes)

- can you explain more ?(5 votes)
- what would you like to know?(5 votes)

- How do you write a sequence function?(3 votes)
- To write an explicit function for the sequence {40, 60, 80, ...}, the sequence increases by 20 each time meaning 'a' will be 20, but 1*20 doesn't equal the starting term so by subtracting 'a' from the starting term you get 'c'. By putting 'a' and 'c' in a format something along the lines of f(n)=an+c, you can write the explicit equation for it.

To write a recursive function, it's similar except now there is another variable. To put it simply, most of the time 'b' is just 1 because most of the time recursive functions define the current term using the previous term. To write a recursive fuction just write something like f(n)=a(n-b)+c.

In non-arithmetic instances of sequences there will be powers and other stuff but I don't know how to explain those without writing a massive chunk of text that will probably be very unclear and almost incomprehensible.(3 votes)

- Can someone explain the domain lesson to me? please HELP!!(2 votes)
- its like the program for the sequence, you tell it what to do, what its job is for(1 vote)

- i'm still not too clear about why there's r^n-1 and r^n in geometric explicit formulas because i see the two a lot in my homework and stuff. could anybody explain, please? thank you :)(2 votes)
- In questions where for example f(1)= -11

why does f(0) work in the final equation? (if the format is f(n) and there is an f(n-1) somewhere in the equation. Wouldn't the value be outside of the domain? the negative first value? I don't get why that works. Why is f(0)=f(0-1)+5 = -11?(2 votes) - Why do you put (n-1) sometimes in parentheses and sometimes as an exponent?(1 vote)
- How do you find the domains of sequences?(1 vote)
- Sequence {0,5,10,15} Domain in sequences are the terms, so here we have 0 as the first term in sequence so the Domain of first term will be A(1) which is 0(0 votes)