Dari suatu barisan ari

u5 = 15
u8 = 24

b = (24-15)/(8-5)
b = 9/3
b = 3

u5 = a + 4b = 15
a + 4x3 = 15
a = 15 - 12
a = 3

<===> a = b = 3
Un = 3n
U20 = 3x20 = 60