bagaimana cara kerjanya...

jawab

u3= 90
u5 = 810


u5/u3 = 810/90
r² = 9
r= 3

u3 = ar^2 =  90
a= 90/r² = 90/9 = 10

sn = a (r^n -1)/(r -1)
s6 =  10 ( 3⁶ - 1)/ (3 -1)
s6= 10/2 (728)  =3.640