SOAL UN SMA, tolong ya pakai carajuga :)

r = 3/2
U5 = a x r⁴ = 81
a x (3/2)⁴ = 81
a x 81/16 = 81
a = 81 x 16/81 = 16

S5 = 16 x ( (3/2)^5 - 1)/(3/2 - 1)
S5 = 16 x 2 x (243/32 - 32/32)
S5 = 32 x (211/32)
S5 = 211 km