C = 81 pi = 2pi r

Hence

2 pi r = 81pi

r = 81 pi / 2 pi

Cancel down

r  [[https://123fox.us/|สล็อตเว็บตรง]] = 40.5

A = pi r^(2)

A = pi ( 40.5)^(2)

A = 1640.25 pi units^(2)