Question

What is the common ratio and explicit formula for -2,6,-18,54?

Answer 1

Answer:

The explicit formula for a given geometric sequence is of the form

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

Step-by-step explanation:

Given sequence is ${\{-2,6,-18,54}\}$

Let $a_{1}=-2,a_{2}=6,a_{3}=-18,a_{4}=54$

To find the common ratio r:

$r=\frac{a_{2}}{a_{1}}$

$=\frac{6}{-2}$

$=-3$

Therefore r=-3

$r=\frac{a_{3}}{a_{2}}$

$=\frac{-18}{6}$

$=-3$

Therefore r=-3

Therefore common ratio is -3

Therefore given sequence is geometric sequence.

The explicit formula for a geometric sequence is $a_{n}=a_{1}^{r-1}$, where common ratio is r.

From given sequence we are having  r=-3

Therefore the explicit formula for a given geometric sequence is of the form $a_{4} =a_{1}^{-3-1}$

$a_{4} =a_{1}^{-4}$

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