tolong dijelasinnnnnn

U5 = a + 4b = 11

U8 + U12 = 52
(a + 7b) + (a + 11b) = 52
2a + 18b = 52
a + 9b = 26

Eliminasi
a + 9b = 26
a + 4b = 11
--------------- --
..... 5b = 15 => b = 3

a + 4b = 11 => a + 4(3) = 11 => a = -1

Sn = n/2 (2a + (n - 1)b)
S8 = 8/2 (2(-1) + 7(3))
= 4 (-2 + 21)
= 4 (19)
= 76

Un = a + (n - 1)b
U5 = a + 4b = 11
U8 = a + 7b
U12 = a + 11b

(a + 7b) + (a + 11b) = 52
(a + a) + (7b + 11b) = 52
2a + 18b = 52

2a + 18b = 52
a + 4b = 11
--------------------- - 
a + 14b = 41
a + 4b = 11
--------------------- - 
10b = 30
b = 30/10
b = 3
a = 11 - 4(3) = 11 - 12 = -1

Jumlah U1 + U2 + ... + U8:
= a + a + b + a + 2b + a + 3b + 11 + a + 5b + a + 6b + a + 7b
= 76

Jawaban : C