tolong bantu ya kakaa kakaa

9.
U3 = a + 2b = 18
U7 = a + 6b = 38
------------------------ -
-4b = -20
b = 5

a + 2b = 18
a + 2x5= 18
a + 10 = 18
a = 8

Sn = n/2[2a+(n-1)b]
S24 = 24/2[2x8 + (24-1)x5]
S24 = 12 ( 16 + 23 x 5)
S24 = 12 (16+ 115)
S24 = 12 x 131
S24 = 1.572 (C)

10.
a = 2
b = 8 - 2 = 6
U20 = 2 + (20-1)x6
U20 = 2 + (19 x 6)
U20 = 2 + 114
U20 = 116 (C)

11.
suku pertama = -3
beda = -5 - (-3)= -5 + 3 = -2
jawaban A

12.
16, 13, 10, 7, 4, ...
beda barisan = 13- 16 = -3
maka suku berikutnya
4 - 3 = 1
1 - 3 = -2
jawaban D