U1=15 U2=45 (pake barisan geometri? Suku ke berapa 729?

[tex]a = 15 \\ ar = 45 \\ r = 3 \\ a {r}^{n - 1} = 729 \\ 15 \times {3}^{n - 1} = {3}^{6} \\ 15 = {3}^{6 - n + 1} \\ 15 = {3}^{7 - n} \\ {3}^{n} = \frac{ {3}^{6} }{5} \\ n = log_{3}( \frac{ {3}^{6} }{5} ) = 6 - log_{3}(5) [/tex]