saya masih belum paham, tolong dibantu

n = 5
U1 = 4
U5 = 324

r^5-1 = U1/U5
r^3 = 324/4
r^3 = 81
r = 3

S5  = a(r^n - 1) / r - 1
      = 4 (3^5 - 1) / 3 -1
      = 4 (243 - 1) / 2
      = 968 /2
      = 484 cm