Diketahui suku ke-7 = 22 dan suku ke-11 = 34. Jumlah 18 suku pertama adalah... Bantu jwb ya :)

Kelas 8 Matematika
Bab Barisan dan Deret Bilangan

b = (U11 - U7)/(11 - 7)
b = (34 - 22)/4
b = 12/4
b = 3

a = U7 - 6b
a = 22 - 6 . 3
a = 22 - 18
a = 4

Sn = n/2 . (2a + (n - 1) . b)

S18 = 18/2 . (2 . 4 + 17 . 3)
S18 = 9 . (8 + 51)
S18 = 9 . 59
S18 = 531

Beda
= (U11-U7) / 11-7
= (34-22) / 4
= 12 / 4
= 3

U7 = a + (n-1)b
22 = a + (7-1)3
22 = a + 18
a = 22 - 18
a = 4

Sn = n/2 (2a + (n-1)b)
S18 = 18/2 (2.4 + (18-1)3)
S18 = 9 (8+ 17.3)
S18 = 9 (8 + 51)
s18 = 9 (59)
S18 = 531

semoga membantu :)