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u4 = ar^3 = 18
2/3 . r^3 = 18
r^3 = 18 x 3/2= 27
r = ∛27 = 3
S5 = a(r^5- 1) / (r-1)
= 2/3 ( 3^5-1) / (3-1)
= 2/3 (243-1) / 2
= 2/6 (242)= 1/3(242)
= 242/3
= 80 2/3
= 80,6667