tolong sama caranya klo bisa

Penyelesaian:

barisan bilangan: 90, 30, 10, ...     (barisan geometri)
a = 90
r = 1/3

masukkan ke rumus Un geometri untuk mencari U4, U5 dan U6, didapat:
Un = a.r^n-1

U4 = 90(1/3)^4-1
= 90(1/3)^3
= 90 . 1/3 . 1/3 . 1/3
= 10/3

U5 = 90(1/3)^5-1
= 90(1/3)^4
= 90 . 1/3 . 1/3 . 1/3 . 1/3
= 10/9

U6 = 90(1/3)^6-1
= 90(1/3)^5
= 90 . 1/3 . 1/3 . 1/3 . 1/3 . 1/3
= 10/27

U4, U5, U6 = 10/3, 10/9, 10/27  (A).