Answer :
The odd-numbered terms are [tex]\dfrac19+\dfrac19[/tex], while the even-numbered terms are [tex]\dfrac19-\dfrac19[/tex], so one formula for the sequence is
[tex]a_n=\dfrac19\left(1+(-1)^{n+1}\right)[/tex]
where [tex]n\ge1[/tex].
[tex]a_n=\dfrac19\left(1+(-1)^{n+1}\right)[/tex]
where [tex]n\ge1[/tex].