tolong dong yang bisa bantu iya beserta caranya

pola : 1/2,5/6,7/6, ......,21/2

maka, a = 1/2
b = 5/6-1/2 = (5-3)/6 = 2/6 = 1/3

Un = 21/2
Un = a + (n-1)b
21/2 = 1/2 + (n-1)1/3
21/2 = 1/2 + 1/3n -1/3
21/2 = 1/3n + 1/2 - 1/3
21/2 = 1/3n + (3-2)/6
21/2 = 1/3n + 1/6
1/3n = 21/2-1/6
1/3n = (63-1)/6
1/3n = 62/6
n = 62/6 . 3/1
n = 31