Diketahui : U2= 18, U5= 42 ditanya : S8 dan S10 dalam barisan aritmatika

Un = a + (n – 1) b
Sn = n/2 (a + Un)

* U2 = a + (2 – 1) b
18 = a + b

*U5 = a + (5 – 1) b
42 = a + 4b

*dieliminasi
a + b = 18
a + 4b = 42
(dikurangi)
– 3b = – 24
b = 8

a + b = 18
a = 18 – 8
a = 10

*U8 = 10 + (8 – 1) 8
= 10 + 56
= 66

S8 = 8/2 (10 + 66)
= 4 x 76
= 304

*U10 = 10 + (10 – 1) 8
= 10 + 72
= 82

S10 = 10/2 (10 + 82)
= 5 x 92
= 460