3. Suku ke-21 barisan 18, 25, 3

Kelas 8 Matematika
Bab Barisan dan Deret Bilangan

18, 25, 34, 45, 58, .... → a = 18
...7, ....9, ...11, ...13, .... → b = 7
......2.......2.....2...... → c = 2

n = 21

Un = a + (n - 1) . b + 1/2 . (n - 1) . (n - 2) . c
U21 = 18 + 20 . 7 + 1/2 . 20 . 19 . 2
U21 = 18 + 140 + 380
U21 = 538

✩✩ A ✩✩