Chapter 8.3, Problem 21E

To determine

To write: the first five terms of the geometric sequence.

Answer to Problem 21E

The first five terms of the geometric sequence are $5,-\frac{1}{2},\frac{1}{20},-\frac{1}{200},\frac{1}{2000}$ .

Explanation of Solution

Given information:

The given sequence,

  $a_1=5,r=-\frac{1}{10}$

Concept used:

The $n$ th term of geometric sequence has the form $a_1=a_1{r}^{n-1}$ where $r$ is the common ratio of the consecutive terms of the sequence.

Every geometric sequence can be written in the following form.

  $a_1=a_1,a_2=a_{1}r,a_3=a_1{r}^2,a_4=a_1{r}^3,\mathrm{...},a_n=a_1{r}^{n-1},\mathrm{..}$

Calculation:

The common ratio and the first term of the sequence is given.

  $\begin{array}{l}{a}_1=5\\ r=-\frac{1}{10}\end{array}$

To find the second term, multiply first term $-\frac{1}{10}$ as shown below.

  $\begin{array}{c}{a}_2=a_1{r}^{2-1}\\ =5\left(-\frac{1}{10}\right)\\ =-\frac{1}{2}\end{array}$

The 3^rd term can be calculated as

  $\begin{array}{c}{a}_3=a_1{r}^{3-1}\\ =5{\left(-\frac{1}{10}\right)}^2\\ =\frac{1}{20}\end{array}$

The 4^th term can be calculated as

  $\begin{array}{c}{a}_4=a_1{r}^{4-1}\\ =5{\left(-\frac{1}{10}\right)}^3\\ =-\frac{1}{200}\end{array}$

The 5^th term can be calculated as

  $\begin{array}{c}{a}_5=a_1{r}^{5-1}\\ =5{\left(-\frac{1}{10}\right)}^4\\ =\frac{1}{2000}\end{array}$

Hence, the first five terms of the geometric sequence are

  $5,-\frac{1}{2},\frac{1}{20},-\frac{1}{200},\frac{1}{2000}$