Suatu barisan aritmatika memiliki u5=120 dan u9=20 nilai u10 adalah

Bab Barisan dan Deret
Matematika SMP Kelas IX

aritmatika
b = (20 - 120)/(9 - 5)
b = -25

U10 = U9 + b
U10 = 20 + (-25)
U10 = -5

Dik:
U5 = 120
a + 4b = 120 ---------------->> (1)

U9 = 20
a + 8b = 20 ------------------>> (2)

DIt:
U10 = ........... ?

Jwb:
Eliminasikan 1 dan 2,,
a + 8b =   20
a + 4b = 120
_____________ -

4b = -100
b = -100/4
b = -25

Masukkan nilai "b" ke 1,,
a + 4b = 120
a + 4(-25) = 120
a + (-100) = 120
a = 120 + 100
a = 220

Jadi,
Un = a + (n-1)b
U10 = 220 + (10-1)-25
U10 = 220 + (9)-25
U10 = 220 + (-225)
U10 = 220 - 225
U10 = -5