Answer:
[tex]A = \frac{1}{2} \pi r [/tex]
multiply all sides by 2:
[tex]2 \times A = ( \frac{1}{2} \pi r) \times 2 \\ \\ 2A = \pi r[/tex]
divide all sides by r:
[tex] \frac{2A}{r} = \frac{\pi r}{r} \\ \\ \frac{2A}{r} = \pi[/tex]
therefore:
[tex]{ \boxed{ \sf{\pi = \frac{2A}{r} }}}[/tex]
