No 8 gan dengan cara

No 8.
deret geometri
a = 5
n = 5
U5 = 80

dit : S5
jawab
U5 = ar^4
80 = 5r^4
16 = r^4
r = 2

Sn = a (r^5 -1)/(r-1)
S5 = 5(2^5 -1)/(2-1)
S5 = 5(32-1)
S5 = 5(31)
S5 = 155 m