Jawabanmu

2014-05-22T15:48:32+07:00
U4 = ar^3 = 81
U6 = ar^5 = 729

U6/U4 = ar^5 / ar^3  = r ^2
729 / 81 = 9 = r^2
r = 3  
r^3 = 27

U4 = ar^3 = 81 = a.27
a = 3

U2 = ar = 9