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a.
r = 6 cm
D = 2 x r = 12 cm
L = 2.π.r.t = 2 x 3,14 x 6 x 12 = 452,16 cm²

b.
D = 8 cm
r = 1/2 D = 4 cm
L = 2.π.r.t = 2 x 3,14 x 4 x 10 = 251,2 cm²

c.
r = 7 cm
D = 2 r = 14 cm
t = L /π.D = 440 / (22/7 x 14) = 440 / 44 = 10 cm

d.
D = L / π.t = 352 / (22/7 x 8) = 352 /(176/7) = 352 x 7/176 = 14 cm
r = 1/2 D = 1/2 x 14 = 7 cm