tolong bantu saya yaa teman teman

Dik :
n = 15
a = 20
b = 28

Dit: Sn?

Jawab:
Sn = n/2 [2a + (n-1)b]
S15 = 15/2 [2.20 + (15-1)8]
S15 = 15/2 [40 + 14.8]
S15 = 15/2 [40 + 112]
S15 = 15/2 x 152
S15 = 1.140 (C)