20,30,42,56,.... U22?

Bab Barisan dan Deret
Matematika SMP Kelas IX

20, 30, 42, 56 → a = 20
   10,  12,  14 → b = 10
        2     2 → c = 2

Un = a + (n - 1) b + 1/2 x (n - 1) x (n -2) x c
U22 = 20 + (21 x 10) + (1/2 x 21 x 20 x 2)
U22 = 20 + 210 + 420
U22 = 650