diketahui barisan aritmatika U5 dari U9 yaitu -8 dan 0 tentukan U10 dan S10

Un = a + ( n - 1) b
U5 = a + 4b = - 8
U9 = a + 8b = 0
-------------------------(-)
- 4b = - 8
b = 2
disubtitusi
a + 4b = - 8
a + 4(2) = - 8
a + 8 = - 8
a = - 8 - 8
a = - 16

U10 = a + 9b
= - 16 + 9(2)
= - 16 + 18
= 2

Sn = ½ . n ( a + Un)
S10 = ½ . 10 ( a + U10)
= 5 ( - 16 + 2)
= 5(-14)
= - 70