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The curved wall of a cylinder unrolls to a 2πr by h rectangle. Adding the two circular ends gives the closed total 2πr² + 2πrh = 2πr(r + h); an open tube omits one or both ends.
Closed cylinder
S = 2πr² + 2πrh = 2πr(r + h)
Open tube
S = 2πrh
Because a tank standing on the ground exposes just the curved wall and the top, not the base it rests on.
2π × 3 × 13 = 78π ≈ 245.044227 square units.