Find the 4th Term -243 , -81 , -27

$- 243$ , $- 81$ , $- 27$

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 $a_{1} = - 243$ and $r = \frac{1}{3}$ .

$a_{n} = - 243 {(\frac{1}{3})}^{n - 1}$

Step 4

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

$a_{n} = - 243 \frac{1^{n - 1}}{3^{n - 1}}$

Step 5

One to any power is one.

$a_{n} = - 243 \frac{1}{3^{n - 1}}$

Step 6

Combine $- 243$ and $\frac{1}{3^{n - 1}}$ .

$a_{n} = \frac{- 243}{3^{n - 1}}$

Step 7

Move the negative in front of the fraction.

$a_{n} = - \frac{243}{3^{n - 1}}$

Step 8

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

$a_{4} = - \frac{243}{3^{(4) - 1}}$

Step 9

Simplify the denominator.

Subtract $1$ from $4$ .

$a_{4} = - \frac{243}{3^{3}}$

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

$a_{4} = - \frac{243}{27}$

Step 10

Simplify the expression.

Divide $243$ by $27$ .

$a_{4} = - 1 \cdot 9$

Multiply $- 1$ by $9$ .

$a_{4} = - 9$

Find the 4th Term -243 , -81 , -27

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