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u4= 40
S6?
Un = ar^n-1
u4= ar^3
40 = 5 x r^3
8 = r^3 --> r = 2
Sn = a(r^n - 1) / (r - 1)
S6 = 5(2^6 - 1) / (2 - 1)
= 5(64 - 1) / 1
= 5(63) --> S6 = 315