Please help to solve this

[mmmt20]

If Bn = (-1)^n * (t)^(n-1)
Col A: B5
Col B: -1+t^2-t^3+t^4
Am I correct in thinking that
B5 = -1*t^4
= -t^4 = a positive no.

What about column B - how do we compare this?
Col B: -1+t^2-t^3+t^4