di ket u1=2/3 barisan geometri dan suku ke tiga =2/27 jumlah 4 suku pertama adalah

U1 = a = 2/3
U3 = ar^2 = 2/27
(a).r^2 = 2/27
(2/3).r^2 = 2/27
r^2 = 2/27 x 3/2
r,^2 = 1/9
r^2 = (1/3)^2
r = 1/3
Sn = a ( 1-r^n) / (1-r)
S4 = 2/3 { 1 - (1/3)^2 } / (1-1/3)
= 2/3 ( 1 - 1/9) / (2/3)
= 1 - 1/9
= 8/9