# Chapter 7 - Exponents and Exponential Functions - 7-8 Geometric Sequences - Practice and Problem-Solving Exercises - Page 470: 26

The explicit formula is $a_{n}$=8 $\times$ (-1$)^{n-1}$

#### Work Step by Step

You have to use the explicit formula. The explicit formula is $a_{n}$=$a_{1}$ $\times$ (r$)^{n-1}$. The starting value is 8 so it is $a_{1}$. You have the sequence 8,-8,8.... so use the common ratio formula(r=$\frac{{a_{2}}}{{a_{1}}}$) r=$\frac{-8}{8}$=-1 .So the common ratio is -1.Substitute the value of a1 and R into the explicit formula: The explicit formula is $a_{n}$=8 $\times$ (-1$)^{n-1}$

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