Dari barisan geometri diket suku ke 2 adl 36 dan suku ke 7 adl 4/27 suku ke 3 barisan tsb adl...

Un = ar^(n-1)
U7 = ar^6 = 4/27
U2 = ar = 36
------------------------ : dibagi
...........r^5 = 1/243
r^5 = (1/3)^5
r = 1/3, substiusi pd slaahsatu
a(1/3) = 36
a = 108
U3 = ar² = 108(1/3)² = 12