# Chapter 7 - Exponents and Exponential Functions - 7-8 Geometric Sequences - Practice and Problem-Solving Exercises - Page 470: 27

The explicit formula is $a_{n}$=686 $\times$ ($\frac{1}{7}$$)^{n-1} #### Work Step by Step You have to use the explicit formula. The explicit formula is a_{n}=a_{1} \times (r)^{n-1}. The starting value is 686 so it is a_{1}. You have the sequence 686,98,14 so use the common ratio formula(r=\frac{{a_{2}}}{{a_{1}}}) r=\frac{686}{98}=\frac{1}{7} and r=\frac{98}{14}=\frac{1}{7}.So the common ratio is \frac{1}{7}.Substitute the value of a1 and R into the explicit formula: The explicit formula is a_{n}=686 \times (\frac{1}{7}$$)^{n-1}$

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