tolong bantu donggg pliss

*a = 5
*U5 = 405
•U5 = ar^n-1
405 = 5.r^4
405/5 = r^4
81 = r^4
3 = r
*S5 = a (r^n - 1) / r-1
= 5 (3^5 - 1) / 3-1
= 5 (243-1) / 2
= 5.242/2
= 605