Tolong dijawab dong no 8 dan 9 (Pakai cara)

Kelas 8 Matematika
Bab Barisan dan Deret Bilangan

8] b = (U9 - U4)/(9 - 4)
b = (28 - 13)/5
b = 15/5
b = 3

a = U4 - 3b
a = 13 - 3 . 3
a = 13 - 9
a = 4

Sn = n/2 . (2a + (n - 1) . b)
S20 = 20/2 . (2 . 4 + 19 . 3)
S20 = 10 . (8 + 57)
S20 = 10 . 65
S20 = 650 

9] a = 1 lembar

r = 2

S5 = ?

Sn = 1 . (rⁿ - 1) / (r - 1)
S5 = 1 . (2⁵ - 1) / (2 - 1)
S5 = (32 - 1)/1
S5 = 31 lembar