diketahui suatu barisan aritmetika dengan U2= 6 dan U7= 31 . suku ke 40 adalah

U2 = 6
a + b = 6 ••• (1)
U7 = 31
a + 6b = 31 ••• (2)

a + 6b = 31
a + b = 6
---------------- -
5b = 25
b = 5

a + b = 6
a + 5 = 6
a = 1

Maka,
U40
= a + 39b
= 1 + 39 (5)
= 1 + 195
= 196

u2 = 6
u7 = 31

a + b = 6
a + 6b = 31
 eliminasi
-5b = -25
b = 5

substitusi
a + b = 6
a +5 = 6
a = 1

u40 = a + 39b
 = 1 + 195
= 196