Question

What is the 9th term of the following geometric sequence? 7/9,-7/3,7,-21,63 ??????

Answer 1

Given:

The geometric sequence is:

$\dfrac{7}{9},\dfrac{-7}{3},7,-21,63,...$

To find:

The 9th term of the given geometric sequence.

Solution:

We have,

$\dfrac{7}{9},\dfrac{-7}{3},7,-21,63,...$

Here, the first term is:

$a=\dfrac{7}{9}$

The common ratio is:

$r=\dfrac{a_2}{a_1}$

$r=\dfrac{\dfrac{-7}{3}}{\dfrac{7}{9}}$

$r=\dfrac{-7}{3}\times \dfrac{9}{7}$

$r=-3$

The nth term of a geometric sequence is:

$a_n=ar^{n-1}$

Where, a is the first term and r is the common ratio.

Substitute $a=\dfrac{7}{9},r=-3,n=9$ to find the 9th term.

$a_9=\dfrac{7}{9}(-3)^{9-1}$

$a_9=\dfrac{7}{9}(-3)^{8}$

$a_9=\dfrac{7}{9}(6561)$

$a_9=5103$

Therefore, the 9th term of the given geometric sequence is 5103.