ditentukan barisan aritmatika U4=9 dan U9=19 . suku ke-30 adalah

U4 = 9 ---> a + 3b = 9
U9 = 19 ---> a + 8b = 19

a + 3b = 9
a + 8b = 19
---------------- -
-5b = -10
b = 2

a + 3 × 2 = 9
a + 6 = 9
a = 3

U30 = a + 29b
= 3 + 29 × 2
= 3 + 58
= 61

Un = a+(n-1)b
U₄ = a+(4-1)b
9 = a+3b ...1
U₉ = a+(9-1)b
19 = a+8b ...2

eliminasi persamaan 1&2
  9 = a+3b
19 = a+8b    - 
-10 = -5b
-10/-5 = b
2 = b

subtitusi b = 2
9 = a+3b
9 = a+3(2)
9 = a+6
9-6 = a
3 = a

Un = a+(n-1)b
U₃₀ = 3+(30-1)2
= 3+(29)2
= 3+58
= 61