tentukan jumlah n suku pertama dari deret geometri berikut! A.8,4,2,1,... B.2,-6,1

A).
8, 4 , 2 , 1, ...
U1 = a = 8
r = U2/U1
r = 4/8 = 1/2

Sn = a(1 - r^n)/(1 - r)
Sn = 8(1 - 1/2 ^n) / (1 - 1/2)
Sn = 8(1 - 2^-n) / (1/2)
Sn = 16 (1 - 2^-n)
Sn = 2^4 (1 - 2^-n)
Sn= 2^4 - 2^4-n

B).
2, -6, 18, -54,...

U1 = a = 2
r = U2/U1
r = -6/2 = -3

Sn = a(1 - r^n)/(1 - r)
Sn = 2(1 - (-3^n)) / ( 1 + 3)
Sn = 2( 1 - (-3^n)) / 4
Sn = 1/2 (1 - (-3^n))

semoga membantu ^-^