diketahui barisan bilangan 3,4,7,12,19... nilai suku ke-12 berdasarkan pola barisan

jawab

3...4...7...12....19
..1...3....5.....7
.....2....2....2

a= 3
b = 1
c = 2

un = a + (n-1)b + 1/2 (c) (n²- 3n + 2)
un = 3 +(n-1)1 + 1/2 (2)(n² - 3n + 2)
un = 3 +n - 1 + n² - 3n + 2
un = n² - 2n + 4
u12 = 12²- 2(12) + 4 = 144 - 24 + 4
u12 = 124