Misalkan Un menyatakan suku ke-n suatu barisan geometri dengan rasio positif. Jika diketahui U8 = 72 dan log U6 + log U8 - log U9 = log 9, maka nilai U7 adalah

Un = ar^(n-1).
U8 = ar7 = 72.....(1)
log U6 + log U8 - log U9 = log 9
log { (U6 × U8) / U9 } = log 9
log ( ar^5 × ar^7 / ar^8 ) = log 9
log (a^2.r^12/ ar^8) = log 9
log (a.r^4) = log 9
a.r^4 = 9....(2)
subsitusikan (2) ke (1)
(1) => ar^7 = 72.
(a.r^4).r^3 = 72. subsitusikan (2) => ar^4 = 9
( 9).r^3 = 72
r^3 = 72/9
r^3 = 8
r^3 = 2^3
r = 2
dari (2)=> ar^4 = 9
a(2)^4 = 9
16a = 9
a = 9/16
U7 = ar^6
= (9/16)(2)^6
= (9/2^4)(2^6)
= 9.2^2
= 9.4
= 36