Chapter 2 - Equations, Inequalities, and Problem Solving - 2.3 Formulas - 2.3 Exercise Set - Page 100: 21

$\pi=\dfrac{A}{r^2}$

$\bf{\text{Solution Outline:}}$ To solve the given equation, $ A=\pi r^2 $ for $ \pi ,$ use the properties of equality to isolate the variable. $\bf{\text{Solution Details:}}$ Using the properties of equality to isolate the variable, the equation above is equivalent to \begin{array}{l}\require{cancel} A=\pi r^2 \\\\ \dfrac{A}{r^2}=\dfrac{\pi r^2}{r^2} \\\\ \dfrac{A}{r^2}=\pi \\\\ \pi=\dfrac{A}{r^2} .\end{array}