Minta bantuannya dong nomor 2 dan 3, besok dikumpulin saya g paham :(

2.)
a = 2/7
r = U2/U1 = 2/21 : 2/7 = 2/21 . 7/2 = 1/3

Un = ar^n-1
Un = 2/7 . (1/3)^n-1

Sn = a(1 - r^n) / 1 - r
Sn = 2/7 (1 - (1/3)^n) / 2/3
Sn = 2/7 (1 - (1/3)^n) . 3/2

3.)
a = 2/3
r = 4/12 : 2/3 = 4/12 . 3/2 = 1/2

Un = ar^n-1
Un = 2/3 . (1/2)^n-1

Sn = a(1 - r^n) / 1 - r
Sn = 2/3 (1 - (1/2)^n) / 1/2
Sn = 2/3 (1 - (1/2)^n) . 2

Semoga membantu dan semoga bermanfaat...