diketahui barisan aritmetika u7 = 19 dan u13 = 37 hitunglah u20

Un = a + ( n - 1 ) b

U7 = a + 6b = 19
U13 = a + 12b = 37
---------------------------(-)
- 6b = - 18
b = 3

a + 6b = 19
a + 6(3) = 19
a + 18 = 19
a = 19 - 18
a = 1

U20 = a + 19b
= 1 + 19(3)
= 1 + 57
= 58