Precalculus Examples

$40$ , $- 10$ , $\frac{5}{2}$ , $- \frac{5}{8}$ , $\frac{5}{32}$

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 $- \frac{1}{4}$ gives the next term. In other words, $a_{n} = a_{1} r^{n - 1}$ .

Geometric Sequence: $r = - \frac{1}{4}$

Step 2

This is the form of a geometric sequence.

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

Step 3

Substitute in the values of $a_{1} = 40$ and $r = - \frac{1}{4}$ .

$a_{n} = 40 {(- \frac{1}{4})}^{n - 1}$

Step 4

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

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Step 4.1

Apply the product rule to $- \frac{1}{4}$ .

$a_{n} = 40 ({(- 1)}^{n - 1} {(\frac{1}{4})}^{n - 1})$

Step 4.2

Apply the product rule to $\frac{1}{4}$ .

$a_{n} = 40 ({(- 1)}^{n - 1} \frac{1^{n - 1}}{4^{n - 1}})$

Step 5

One to any power is one.

$a_{n} = 40 ({(- 1)}^{n - 1} \frac{1}{4^{n - 1}})$

Step 6

Combine ${(- 1)}^{n - 1}$ and $\frac{1}{4^{n - 1}}$ .

$a_{n} = 40 \frac{{(- 1)}^{n - 1}}{4^{n - 1}}$

Step 7

Combine $40$ and $\frac{{(- 1)}^{n - 1}}{4^{n - 1}}$ .

$a_{n} = \frac{40 {(- 1)}^{n - 1}}{4^{n - 1}}$