Chapter 8.4, Problem 34E

To determine

Check whether the given series is arithmetic, geometric or neither.

Answer to Problem 34E

Thegiven series is an arithmetic.

Explanation of Solution

Given:

The given series is $-7,-1,5,11,17,\mathrm{...}$

Calculation:

The general rule for the $n\text{th}$ term is $a_n=a_1+\left(n-1\right)d$ .

Where $a_1\to$ first term, $d\to$ common difference, $n\to$ number of terms.

In the given sequence $-7,-1,5,11,17,\mathrm{...}$

  $\begin{array}{l}{a}_2-a_1=-1-\left(-7\right)=6\\ a_3-a_2=5-\left(-1\right)=6\\ a_4-a_3=11-5=6\\ a_5-a_4=17-11=6\end{array}$

The common difference $\left(6\right)$ is constant.

In a geometric sequence, the ratio of any term to the previous term, called the common ratio, is constant.

Find the common ratio of any term to the previous term of the sequence $-7,-1,5,11,17,\mathrm{...}$

  $\begin{array}{l}\frac{a_2}{a_1}=\frac{-1}{7}\\ \frac{a_3}{a_2}=\frac{5}{-1}=-5\\ \frac{a_2}{a_1}\ne \frac{a_3}{a_2}\end{array}$

The common ratio is not constant.

Hence the given series is an arithmetic.