Question

A geometric sequence is defined by the equation an = (3)3 − n.

Answer 1

PART A

The geometric sequence is defined by the equation

$a_{n}=3^{3-n}$

To find the first three terms, we put n=1,2,3

When n=1,

$a_{1}=3^{3-1}$

$a_{1}=3^{2}$

$a_{1}=9$
When n=2,

$a_{2}=3^{3-2}$
$a_{2}=3^{1}$

$a_{2}=3$

When n=3

$a_{3}=3^{3-3}$

$a_{3}=3^{0}$
$a_{1}=1$
The first three terms are,

$9,3,1$

PART B

The common ratio can be found using any two consecutive terms.

The common ratio is given by,
$r= \frac{a_{2}}{a_{1}}$
$r = \frac{3}{9}$

$r = \frac{1}{3}$

PART C

To find
$a_{11}$

We substitute n=11 into the equation of the geometric sequence.

$a_{11} = {3}^{3 - 11}$

This implies that,

$a_{11} = {3}^{ - 8}$

$a_{11} = \frac{1}{ {3}^{8} }$

$a_{11}=\frac{1}{6561}$