hitung jumlah lima suku pertama (u1=512 dan u3=128)

U1 = a = 512
U3 = ar² = 128

ar² = 128
(512)r² = 128
r² = 128/512
r² = 1/4
r = ²√1/4
r = 1/2

S5 = 512 (1 - (1/2)⁴) / (1 - 1/2)
S5 = 512 (1 - 1/16) / (1/2)
S5 = 512 (15/16) / (1/2)
S5 = 480 / (1/2)
S5 = 960