Solve for r
r=\frac{c}{2\pi }
Solve for c
c=2\pi r
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2\pi r=c
Swap sides so that all variable terms are on the left hand side.
\frac{2\pi r}{2\pi }=\frac{c}{2\pi }
Divide both sides by 2\pi .
r=\frac{c}{2\pi }
Dividing by 2\pi undoes the multiplication by 2\pi .
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