Find the number of term s in the following arithmetic sequence.

Question

3, 9, 15, 21... 123

Answer 1

a = 3 , d = 6

term(n) = a + (n-1)d
123 = 3 + (n-1)(6)
120 = 6n - 6
126 = 6n
n = 126/6 = 21

there are 21 terms