Find the sum of the first 9 terms of the following sequence. Round to the nearest hundredth if necessary. 15 , − 90 , 540 , . . . 15,−90,540,... Sum of a finite geometric series: Sum of a finite geometric series: S n = a 1 − a 1 r n 1 − r S n ​ = 1−r a 1 ​ −a 1 ​ r n ​