diketahui U6= 19 U8=25 ditanya U21 beserta jalannya

Un = a + (n - 1) b

U6 = a + 5b = 19
U8 = a + 7b = 25
--------------------------(-)
- 2b = - 6
b = 3

a + 5b = 19
a + 5(3) = 19
a + 15 = 19
a = 19 - 15
a = 4

U21 = a + (n - 1)b
= 4 + (21 - 1) 3
= 4 + 20 × 3
= 4 + 60
= 64