Tentukan jumlah 8 suku dari deret 54 + 18 + 6 + ..... Tolong beri caranya

a = 54, r = 18/54 = 6/18 = 1/3
Sn = a (1 - r^n) / ( 1 - r ), |r| < 181
S8 = 54 ( 1 - (1/3)8 ) / (1 - 1/3)
= 54 ( 1 - 1/6561 ) / 2/3
= 54 ( 6560/ 6561 ) / 2/3
= 81 ( 6560/6561)
= 531.560 / 6561
= please you look for alone