suku ke 5 dan suku ke 8 barisan Aritmetika berturut -turut 14 dan 23 tentukan ke 30:)

U5 = 14
U8 = 23
U30 = ...?
Jawab :
U5 = 14
a + 4b = 14 ··· (1)

U8 = 23
a + 7b = 23 ··· (2)

a + 4b = 14
a + 7b = 23
_________ --
.... -3b = -9
....... b = -9/-3
....... b = 3

a + 4b = 14 ··· (1)
a + 4.3 = 14
a + 12 = 14
a = 14 - 12
a = 2

Maka,
Un = a + ( n - 1 ) b
U30 = 2 + ( 30 - 1 ) 3
U30 = 2 + 87
U30 = 89