Find the 5th Term -324 , 108 , -36 , 12

$- 324$ , $108$ , $- 36$ , $12$

Step 1

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 $- \frac{1}{3}$ gives the next term. In other words, $a_{n} = a_{1} r^{n - 1}$ .

Geometric Sequence: $r = - \frac{1}{3}$

Step 2

This is the form of a geometric sequence.

$a_{n} = a_{1} r^{n - 1}$

Step 3

Substitute in the values of $a_{1} = - 324$ and $r = - \frac{1}{3}$ .

$a_{n} = - 324 {(- \frac{1}{3})}^{n - 1}$

Step 4

Use the power rule ${(a b)}^{n} = a^{n} b^{n}$ to distribute the exponent.

Apply the product rule to $- \frac{1}{3}$ .

$a_{n} = - 324 ({(- 1)}^{n - 1} {(\frac{1}{3})}^{n - 1})$

Apply the product rule to $\frac{1}{3}$ .

$a_{n} = - 324 ({(- 1)}^{n - 1} \frac{1^{n - 1}}{3^{n - 1}})$

Step 5

One to any power is one.

$a_{n} = - 324 ({(- 1)}^{n - 1} \frac{1}{3^{n - 1}})$

Step 6

Combine ${(- 1)}^{n - 1}$ and $\frac{1}{3^{n - 1}}$ .

$a_{n} = - 324 \frac{{(- 1)}^{n - 1}}{3^{n - 1}}$

Step 7

Combine $- 324$ and $\frac{{(- 1)}^{n - 1}}{3^{n - 1}}$ .

$a_{n} = \frac{- 324 {(- 1)}^{n - 1}}{3^{n - 1}}$

Step 8

Move the negative in front of the fraction.

$a_{n} = - \frac{324 {(- 1)}^{n - 1}}{3^{n - 1}}$

Step 9

Substitute in the value of $n$ to find the $n$ th term.

$a_{5} = - \frac{324 {(- 1)}^{(5) - 1}}{3^{(5) - 1}}$

Step 10

Simplify the numerator.

Subtract $1$ from $5$ .

$a_{5} = - \frac{324 {(- 1)}^{4}}{3^{5 - 1}}$

Raise $- 1$ to the power of $4$ .

$a_{5} = - \frac{324 \cdot 1}{3^{5 - 1}}$

Step 11

Simplify the denominator.

Subtract $1$ from $5$ .

$a_{5} = - \frac{324 \cdot 1}{3^{4}}$

Raise $3$ to the power of $4$ .

$a_{5} = - \frac{324 \cdot 1}{81}$

Step 12

Simplify the expression.

Multiply $324$ by $1$ .

$a_{5} = - \frac{324}{81}$

Divide $324$ by $81$ .

$a_{5} = - 1 \cdot 4$

Multiply $- 1$ by $4$ .

$a_{5} = - 4$

Find the 5th Term -324 , 108 , -36 , 12

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