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U4 = ar³
18 = 2/3r³
18x3/2 = r³
27 = r³
r = 3
s4 = a(r^4-1)/(r-1)
S4 = 2/3 (3^4 -1) / (3-1)
s4 = 2/3 (81-1) /2
s4 = 2/3 . 80 / 2
s4 = 80/3