suatu barisan aritmetika mempunyai u5= 8 dan u9= 20 . maka suku ke-25 adalah

BARISAN DAN DERET
Aritmetika
U5 = a+4b = 8
U9 = a+8b = 20

»U9–U5
(a+8b)–(a+4b) = 20–8
4b = 12
b = 12/4
b = 3

» U5 = a+4b
a+(4×3) = 8
a+12 = 8
a = 8–12
a = -4

»U25 = a+24b
U25 = –4+(24×3)
U25 = –4+72
U25 = 68

u5 = 8
u9 = 20

u5 = a + 4b
U9 = a + 8b

a+ 4b = 8
a + 8b = 20

eliminasi

-4b = -12
b = 3

a + 8b = 20
a + 24 = 20
a = 20 -24
a = -4

u25 = a + 24b
       = -4 + 24 (3)
       = -4 + 72
       = 68