hitunglah jumlah 8 suku pertama dari deret 5+20+80+320+...!

jawab

a= 5
r= 20/5 = 4

sn  =  a (r^n  -1)/(r -1)
sn = 5 (4^8 - 1)/(4 -1)
s8 = 5/3 (65535) = 109.225

a = 5
r = 20/5 = 4
n = 8

Dn = [a(1 - r^n)]/[1 - r]
D8 = [5(1 - 4^8)]/[1 - 4]
D8 = [5(1 - 65536)]/[-3]
D8 = [5(-65535)]/[-3]
D8 = -327675/(-3)
D8 = 109225