suku ke-9 dan suku-13 barisan aritmetika berturut- turut 18 dan 34. nilai suku ke-18 barisan tersebut adalah ?

Un = bn + a - b
U9 = 9b + a - b
18 = 8b + a

Un = bn + a - b
U13 = 13b + a - b
34  = 12b + a

18 = 8b + a
34 = 12b + a
------------------ -
-16 = -4b
   b = -16/-4
   b = 4

18 = 8b + a
18 = 8.4 + a
18 = 32 + a
  a = 18 - 32
  a = -14

Un = bn + a - b
U18 = 4.18 + (-14) - 4
        = 72 - 18
        = 54

U9 = a + 8b = 18
U13= a + 12b = 34 -
------------------------ dieliminasi
4b = 16
b = 4

a+8(4)=18
a+ 32= 18
a= -14

U18 = a+17b
= -14 + 17(4)
= -14 + 68
= 54