U4=25 dan U9=45. Tentukan suku ke-7 dari barisan aritmatika

Diketahui :
U4 = 25
U9 = 45

Ditanya :
U7 = ?

Jawab :
U4 = a + (4-1) b
25 = a + 3b........... pers 1

U9 = a + (9-1) b
45 = a + 8b...........pers 2

Eliminasi :
a + 3b = 25
a + 8b = 45
------------------ -
-5b = - 20 (dibagi -5)
b = 4

Subtitusi b = 4 ke pers 1 :
25 = a + 3b
25 = a + 3 (4)
25 = a + 12
25-12 = a
13 = a

U7 = a + (n-1) b
= 13 + (7-1) 4
= 13 + 6 (4)
= 13 + 24
= 37