tolong bantu no .10 y beserta caranya

Dik. n=5
       U1=5
       Un=405
Dit. Sn= ?
Jwb.
Un=U1.r^n-1
405=5.r^5-1
405=5.r^4
r^4=81
r=3

Sn=U1.(r^n -1)/r-1
Sn= 5(3^5 -1)/3-1
Sn=5(243-1)/2
Sn=5.242/2
Sn=5.121
     = 605cm