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2πr² + 2πr.9 = 3.960
2π(r² + 9r) = 3.960
2(22/7)(r² + 9r) = 3.960
r² + 9r = 3.960(7/44)
r² + 9r = 630
r² + 9r - 630 = 0
(r + 30)(r - 21) = 0
r₁ = -30 r₂ = 21
kita gunakan r₂ karena hasilnya positif
diameter tabung = 21 x 2 = 42 cm