Solve for a
\left\{\begin{matrix}a=\frac{A}{\pi r}\text{, }&r\neq 0\\a\in \mathrm{R}\text{, }&A=0\text{ and }r=0\end{matrix}\right.
Solve for A
A=\pi ar
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\pi ra=A
Swap sides so that all variable terms are on the left hand side.
\frac{\pi ra}{\pi r}=\frac{A}{\pi r}
Divide both sides by \pi r.
a=\frac{A}{\pi r}
Dividing by \pi r undoes the multiplication by \pi r.
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