Tolong jawab pakai cara ya

Mapel = Matematika
Bab = Barisan dan Deret
Tentang = Deret Aritmetika

Diketahui
U3 = 8
U8 = 23

Beda = (U8 - U3) / (8-3)
Beda = (23 - 8) / 5
Beda = 15/5
Beda = 5

Un = a + (n-1)b
U3 = a + (3-1)5
8 = a + 10
-a = 10 - 8
-a = 2
a = -2

a. Suku ke 15
Un = a + (n-1)b
U15 = -2 + (15-1)5
U15 = -2 + 70
U15 = 68

b. Jumlah 20 suku pertama
Sn = n/2 (2a + (n-1)b)
S20 = 20/2 (2 . -2 + (20-1)5)
S20 = 10 (-4 + 95)
S20 = 10 (91)
S20 = 910

Semoga membantu :)