2,6,10,14,........un..... u18

Materi Barisan dan Deret
Diketahui a = 2 dan b = 6 - 2 = 4.
[tex] u_n = a + (n - 1)b = 2 + (n-1) \times 4 = 2 + 4n - 4 = 4n - 2 \\ u_{18} = 4(18)-2 = 70 [/tex]