# How to write a geometric sequence formula

If we simplify that equation, we can find a1. However, we do know two consecutive terms which means we can find the common ratio by dividing. Site Navigation Geometric Sequences This lesson will work with arithmetic sequences, their recursive and explicit formulas and finding terms in a sequence.

This will give us Notice how much easier it is to work with the explicit formula than with the recursive formula to find a particular term in a sequence.

Your formulas should be simplified if possible, but be very careful when working with exponential expressions. If we do not already have an explicit form, we must find it first before finding any term in a sequence.

If you need to review these topics, click here. What happens if we know a particular term and the common ratio, but not the entire sequence? Now that we know the first term along with the r value given in the problem, we can find the explicit formula.

Notice that the an and n terms did not take on numeric values. Now we use the formula to get Notice that writing an explicit formula always requires knowing the first term and the common ratio.

There must be an easier way. What is your answer? To find the 10th term of any sequence, we would need to have an explicit formula for the sequence.

In this lesson, it is assumed that you know what an arithmetic sequence is and can find a common difference. Given the sequence 2, 6, 18, 54.

However, the recursive formula can become difficult to work with if we want to find the 50th term. Find a6, a9, and a12 for problem 6. Find the explicit formula for a geometric sequence where and. Find a6, a9, and a12 for problem 8. You must substitute a value for r into the formula.

In this situation, we have the first term, but do not know the common ratio. Using the recursive formula, we would have to know the first 49 terms in order to find the 50th. We have r, but do not know a1. For example, when writing the general explicit formula, n is the variable and does not take on a value.

Find the recursive formula for 0. To write the explicit or closed form of a geometric sequence, we use anis the nth term of the sequence.

You will either be given this value or be given enough information to compute it. But if you want to find the 12th term, then n does take on a value and it would be The formula says that we need to know the first term and the common ratio.

This sounds like a lot of work. Find the explicit formula for 5, 10, 20, 40. The first time we used the formula, we were working backwards from an answer and the second time we were working forward to come up with the explicit formula. Find the explicit formula for 0.

The first term in the sequence is 2 and the common ratio is 3. So the explicit or closed formula for the geometric sequence is. Find the recursive formula for 5, 10, 20, 40. The recursive formula for a geometric sequence is written in the form For our particular sequence, since the common ratio r is 3, we would write So once you know the common ratio in a geometric sequence you can write the recursive form for that sequence.

If neither of those are given in the problem, you must take the given information and find them. Since we already found that in our first example, we can use it here.

Rather than write a recursive formula, we can write an explicit formula. Look at the example below to see what happens. Find a6, a9, and a12 for problem 4. Notice this example required making use of the general formula twice to get what we need. This geometric sequence has a common ratio of 3, meaning that we multiply each term by 3 in order to get the next term in the sequence.So this is a geometric series with common ratio r = –2.

(I can also tell that this must be a geometric series because of the form given for each term: as the index increases, each term will be multiplied by an additional factor of –2.).

The first term of the sequence is a = –mint-body.comng into the summation formula, I. Geometric sequences calculator that shows all the work, detailed explanation and steps. Site map; Math Tests; Geometric sequence is a list of numbers where each term is obtained by multiplying the previous term by a constant.

probably have some question write me using the contact form or email me on Send Me A Comment. Comment: Email. Write the arithmetic sequence formula that represents the sequence below. 3, 8, 13, 18, 23, You may be interested in the Arithmetic Sequence Formula Videos below. a n is the nth term of the sequence.

When writing the general expression for a geometric sequence, you will not actually find a value for this. It will be part of your formula much in the same way x’s and y’s are part of algebraic equations. Using Explicit Formulas for Geometric Sequences Because a geometric sequence is an exponential function whose domain is the set of positive integers, and the common ratio is the base of the function, we can write explicit formulas that allow us to find particular terms.

Use the formula for the sum of a geometric series to determine the sum when a 1 =4 and r=2 and we have 12 terms.

How to write a geometric sequence formula
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