(deretan geometri) tolong dibantu dengan cara

n = 5, a = 5 dan U5 = 405
Un = ar^(n-1)
U5 = ar^4
405 = 5r^4
r^4 = 81
r^4 = 3^4
r = 3
Sn = a(r^n - 1)/(r - 1)
S5 = 5(3^5 - 1)/(3 - 1) = 605