tentukan banyak suku barisan geometri 1,2,4,8...256

a=1
r=2
ditanya Sn
Un=ar^(n-1)
256= 1. 2^(n-1)
2^8 = 2^(n-1)
8 =n-1
n = 9
Sn = a (r^n - 1)
= 1 (2^9 -1)
= 512-1
= 511
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