Diketahui suatu barisan aritmetika dengan U5 = 7 dan U8 = 13. Suku ke-20 adalah….

U5 = a + 4b
7 = a + 4b •••• (1)
U8 = a + 7b
13 = a + 7b ••• (b)

a + 4b = 7
a + 7b = 13
----------------- -
-3b = -6
b = 2

a + 4b = 7
a + 4 (2) = 7
a + 8 = 7
a = -1

Maka,
U20
= a + 19b
= -1 + 19 (2)
= -1 + 38
= 37