a<sub>n</sub> = (-1)^n*(n^3)/n^3 +2n^2+1
doesn't make sense. it =
(-1)^n + 2n^2+1
so I assume you mean
a<sub>n</sub> = (-1)^n*(n^3)/(n^3 +2n^2+1)
so i divided top and bottom by n^3 to get (-1)^n/(1 + 2/n + 1/n^3)
which approaches
...+1,-1,+1,-1,+1...
BTW I used a<sub>n</sub> to get a<sub>n</sub>