Chapter 6 - Section 6.6 - Rational Equations and Problem Solving - Exercise Set - Page 387: 16

$a_1=S(1-r)+a_nr$

Using the properties of equality, in terms of $ a_1 ,$ the given equation, $ S=\dfrac{a_1-a_nr}{1-r} ,$ is equivalent to \begin{array}{l} S(1-r)=a_1-a_nr \\\\ S(1-r)+a_nr=a_1 \\\\ a_1=S(1-r)+a_nr .\end{array}