Question

Is the sequence 5,9,15,..an arthmetric sequence explain

Answer 1

Is not because  9 - 5 = 4; 15 - 9 = 6; 4 is not equal with 6!

Answer 2

If a, b, c is an arithmetic sequence then a + c = 2b

a = 5, b = 9, c = 15

subtitute

a + c = 5 + 15 = 20
2b = 2 · 9 = 18

20 ≠ 18

Answer: The sequence 5, 9, 15 is not an arithmetic sequence.