The sequence is not an arithmetic sequence because. The n-term for an arithmetic sequence is:
[tex]a_n\text{ = }a_1\text{ + }(n-1)d[/tex]where d is the difference between two consecutive numbers. In this case d = 2. Because
15-13 = 2
13-11 = 2
11- 9 = 2
now, the first term for 15,13,11,9 is 15 = a_1, the second term would be:
[tex]a_2\text{ = }a_1\text{ + }(2-1)d\text{ }[/tex]that is equivalent to say:
[tex]a_2\text{ = 15 + }(2)2=19\ne13\text{ }[/tex]So we can conclude that 15,13,11,9 is not an arithmetic sequence.
