diketahui barisan aritmetika suku 4=3 suku ke 7=-6 hitunglah suku ke 12 dan jumlah 12 suku pertama

Kelas 8 Matematika
Bab Barisan dan Deret Bilangan

b = (U7 - U4)/(7 - 4)
b = (-6 - 3)/3
b = -9/3
b = -3

a = U4 - 3b
a = 3 - 3 . (-3)
a = 3 + 9
a = 12

1] U12 = U7 + 5b
U12 = -6 + 5 . (-3)
U12 = -6 - 15
U12 = -21

2] Sn = n/2 . (a + Un)

S12 = 12/2 . (12 + (-21))
S12 = 6 . (12 - 21)
S12 = 6 . (-9)
S12 = -54