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a . r = 18
r = 18 : 6 = 3
Sn = a(r ⁿ - 1) / r - 1
S₅ = 6(3 ⁵ - 1) / 3 - 1
S₅ = 6(243 - 1) / 2
S₅ = 6(242) / 2
S₅ = 726