Question

-5, 20, -80, 320, ... is a geometric sequence a. Find the common ratio, b. Write its recursive rule c. Write its explicit rule.What is the common ratio and explicit rule?

Answer 1

a.

In order to find the common ratio, we just need to divide a term by the term that comes before it.

So using the terms 20 and -5, we have:

$\text{ratio}=\frac{20}{-5}=-4$

b.

The recursive rule can be found with the formula:

$a_n=a_{n-1}\cdot q$

Where an is the nth term and q is the ratio. So we have:

$a_n=a_{n-1}\cdot(-4)$

c.

The explicit rule can be written as:

$a_n=a_1\cdot q^{n-1}$

Where an is the nth term, a1 is the first term and q is the ratio. So:

$a_n=-5\cdot(-4)^{n-1}$