Suatu barisan aritmatika dengan U5 = -3, U8+U15=20. Tentukan: a, b dan U27.

Un = a + (n - 1)b

U5 = a + 4b = -3 
U8 + U15 = (a + 7b) + (a + 14b) = 2a + 21b = 20

a + 4b = -3             x2
2a + 21b = 20        x1
_________________
2a + 8b = -6
2a + 21b = 20
______________-
-13b = -26
b = 2

a + 4b = -3
a + 8 = -3
a = -11

U27 = a + 26b = -11 + 26(2) = -11 + 52 = 41 

a = -11
b = 2
U27 = 41