Bagaimana caranya??? Bantu saya yaa

barisan geometri 4 suku
Un = ar^(n-1)
U1 = a = 6
U4 = ar^3 = 48
ar^3 = 48
6r^3 = 48
r^3 = 48/6
r^3 = 8
r^3 = 2^3
r = 2

panjang tali mula-mula : S4
S4 = (a(r^n -1))/r-1
S4 = (6(2^4 -1))/2-1
S4 = 6(16-1)/1
S4 = (6x15)
S4 = 90