toling di jawab nomor 8 dan bagaimana cara pengerjaanya?

Kelas 8 Matematika
Bab Barisan dan Deret Bilangan

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