
Yefim S. answered 07/09/23
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C = 2πr; S = 4πr2; dS = 8πrdr; dC = 2πdr; dr = dC/(2π), r = C/(2π)·. So, dS = 8π·C/(2π)·dC/(2π) = 2CdC/π =
2·70·0.5/π cm2 = 22.282 cm2