nth Term of an Geometric Sequence – Examples and Practice

Any term of a geometric sequence can be found by using the value of the common ratio, the position of the term, and the value of the first term of the sequence. With these values, we can form a formula to find the nth term.

Here, we will look at some solved examples of the nth term of a geometric sequence. In addition, we will see some practice problems.

ALGEBRA
Formula for the nth term of a geometric sequence

Relevante para

Solving some examples of geometric sequences.

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ALGEBRA
Formula for the nth term of a geometric sequence

Relevant for

Solving some examples of geometric sequences.

See examples

10 Examples of the nth term of geometric sequences with answers

If you need it, you can look at a revision of the formulas for the nth term of geometric sequences in this article.

EXAMPLE 1

Find the 4th term of a geometric sequence where the first term is 5 and the common ratio is 2.

In this case, we know the values of the first term and the common ratio directly. Then, we can see the following information:

  • First term $latex a=5$
  • Common ratio: $latex r=2$
  • Position of term: $latex n=4$

Now, we apply the formula for the nth term of a geometric sequence with the given values:

$latex a_{n}=ar^{n-1}$

$latex a_{4}=(5)(2)^{4-1}$

$latex a_{4}=(5)(2)^3$

$latex a_{4}=(5)(8)$

$latex a_{4}=40$

EXAMPLE 2

What is the 6th term of a geometric sequence in which the first term is 3 and the common ratio is 3?

As in the previous example, here we also know the values of the first term and the common ratio. Then, we have:

  • $latex a=3$
  • $latex r=3$
  • $latex n=6$

When we use the formula for the nth term of a geometric sequence, we have:

$latex a_{n}=ar^{n-1}$

$latex a_{6}=(3)(3)^{6-1}$

$latex a_{6}=(3)(3)^5$

$latex a_{6}=(3)(243)$

$latex a_{6}=729$

EXAMPLE 3

Find the value of the 6th term of a geometric sequence where the first term is 6 and the common ratio is -2.

We can observe the following values:

  • $latex a=6$
  • $latex r=-2$
  • $latex n=6$

We see that we have a negative common ratio. However, the formula for the nth term applies in any case:

$latex a_{n}=ar^{n-1}$

$latex a_{6}=(6)(-2)^{6-1}$

$latex a_{6}=(6)(-2)^5$

$latex a_{6}=(6)(-32)$

$latex a_{6}=-192$

EXAMPLE 4

The first four terms of a geometric sequence are 3, 6, 12, and 24. What is the value of the 6th term of the sequence?

In this example, we know the value of the first term, but we don’t know the value of the common ratio of the sequence.

We can find the value of the common ratio by dividing any term by its previous term. For example, $latex \frac{6}{3}=2$. Therefore, we have:

  • $latex a=3$
  • $latex r=2$
  • $latex n=6$

Now, we use these values in the formula for the nth term:

$latex a_{n}=ar^{n-1}$

$latex a_{6}=(3)(2)^{6-1}$

$latex a_{6}=(3)(2)^5$

$latex a_{6}=(3)(32)$

$latex a_{6}=96$

EXAMPLE 5

Find the 6th term of a geometric sequence that starts with the terms -5, 15, -45, …

We have to start by finding the value of the common ratio. Then, we have $latex \frac{15}{-5}=-3$:

  • $latex a=-5$
  • $latex r=-3$
  • $latex n=6$

Now, we can apply the formula for the nth term:

$latex a_{n}=ar^{n-1}$

$latex a_{6}=(-5)(2)^{6-1}$

$latex a_{6}=(-5)(-3)^5$

$latex a_{6}=(-5)(-243)$

$latex a_{6}=1215$

EXAMPLE 6

Find the 10th term of a geometric sequence that starts with the terms 2, 4, 8, …

The common ratio of the progression is $latex \frac{4}{2}=2$. Then, we have the following values:

  • $latex a=2$
  • $latex r=2$
  • $latex n=10$

Now, we can apply the formula for the nth term with these values:

$latex a_{n}=ar^{n-1}$

$latex a_{10}=(2)(2)^{10-1}$

$latex a_{10}=(2)(2)^9$

$latex a_{10}=(2)(512)$

$latex a_{10}=1024$

EXAMPLE 7

If a geometric sequence begins with terms 1, -3, 9, …, what is the value of the 7th term?

The value of the common ratio of the sequence is $latex \frac{-3}{1}=-3$. Then, we have the following:

  • $latex a=1$
  • $latex r=-3$
  • $latex n=7$

Now, we apply the formula for the nth term with the given values:

$latex a_{n}=ar^{n-1}$

$latex a_{7}=(1)(-3)^{7-1}$

$latex a_{7}=(1)(-3)^6$

$latex a_{7}=(1)(729)$

$latex a_{7}=729$

EXAMPLE 8

What is the 9the term of a geometric sequence that starts with the terms 2, 3, $latex 4\frac{1}{2}$?

We find the common ratio by dividing a term by its previous term: $latex \frac{3}{2}$. Therefore, we have:

  • $latex a=2$
  • $latex r=\frac{3}{2}$
  • $latex n=9$

Now, we use the formula for the nth term:

$latex a_{n}=ar^{n-1}$

$$a_{9}=(2)\left(\frac{3}{2}\right)^{9-1}$$

$$a_{9}=(2)\left(\frac{3}{2}\right)^8$$

$$a_{9}=(2)\left(\frac{6561}{256}\right)$$

$$a_{9}=\frac{6561}{128}$$

$$a_{9}=51\frac{33}{128}$$

EXAMPLE 9

The first three terms of a geometric sequence are 81, -54, 36, … What is the value of the 8th term?

The common ratio of the sequence is $latex \frac{-54}{81}=-\frac{2}{3}$. Then, we have the following values:

  • $latex a=81$
  • $latex r=-\frac{2}{3}$
  • $latex n=8$

Applying the formula for the nth term with the given values, we have:

$latex a_{n}=ar^{n-1}$

$$a_{8}=(81)\left(-\frac{2}{3}\right)^{8-1}$$

$$a_{8}=(81)\left(-\frac{2}{3}\right)^7$$

$$a_{8}=(81)\left(-\frac{128}{2187}\right)$$

$$a_{8}=-\frac{128}{27}$$

$$a_{8}=-4\frac{20}{27}$$

EXAMPLE 10

Find the 5th term of a geometric sequence that starts with the terms $latex 2, ~\frac{2}{5}, ~\frac{2}{25}$, …

The common ratio of the sequence is $latex \frac{2}{5}\div 2= \frac{1}{5}$. Therefore, we have the following:

  • $latex a=2$
  • $latex r=\frac{1}{5}$
  • $latex n=5$

Now, we use these values in the formula for the nth term of a geometric sequence:

$latex a_{n}=ar^{n-1}$

$$a_{5}=(2)\left(\frac{1}{5}\right)^{5-1}$$

$$a_{5}=(2)\left(\frac{1}{5}\right)^4$$

$$a_{5}=(2)\left(\frac{1}{625}\right)$$

$$a_{5}=\frac{2}{625}$$


nth Term of geometric sequences – Practice problems

nth term of geometric sequences quiz
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Find the value of the 100th term of the geometric sequence that begins with the terms 7, -7, 7, …

Write the answer in the input box.

$latex a_{100}=$

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