soal matematika kelas9

a + 2b = 10
a + 6b = 22
___________-
---- -4b = -12
------- b = 3


b = 3 Substitusi ke

a + 2b = 10
a + 2.3 = 10
a = 10 - 6
a = 4

Sn = n/2 (2a + (n - 1)b
S30 = 30/2 (2.4 + (30 - 1) 3
S30 = 15 (8 + (29.3)
S30 = 15 × 95
S30 = 1.425


jadi jawabannya adalah D