soal geometri tentukan jumlah 8 suku pertama dari deret A.1+3+9+27+... B.-8+4+(-2)+1+(-1/2)+...

A.
a = 1
r = 3/1 = 3
S8 = 1 x (3^8 - 1)/(3-1) = 6560/2 = 3.280

B.
a = -8
r = 4/-8 = -½
S8 = -8 x ( 1 - (-½)^8)/(1-(-½))
= -8 x (1 - 1/256)/(1+½)
= -8 x 255/256 x 2/3
= -4.080/768
= -5,3125