1. 2,5,9,14,20 Un = ....... ? U15=....... ? mohon bantuannya

2,5,9,14,20
a=2 , b=5-2=3
Un=a+(n-1)b
Un=2+(n-1)3
Un=2+(3n-3)

U15=a+(n-1)b
U15=2+(15-1)3
U15=2+(14)3
U15=2+42
U15=44

Semoga membantu..

Bab Barisan dan Deret
Matematika SMP Kelas IX

2, 5, 9, 14, 20, .... → a = 2
  3, 4, 5,  6, ... → b = 3
     1, 1, 1,.. → c = 1

A] Un = a + (n - 1) b + 1/2 x (n - 1) (n - 2) x c
Un = 2 + (n - 1) 3 + 1/2 x (n² - 3n + 2) x 1
Un = 2 + 3n - 3 + 1/2 (n² - 3n + 2)
Un = 3n - 1 + 1/2 (n² - 3n + 2)
Un = (n² + 6n - 3n + 2 - 2)/2
Un = (n² + 3n)/2

B] U15 = (15² + 3 x 15)/2
U15 = (225 + 45)/2
U15 = 135