I fear I am missing something given the wording Ady has used but here goes:

1) Precision does matter because the limit of the function as x--> +infinity is 1.0.  If you use a calculator, program or spread sheet, you could get 1.0 as an answer for some of the higher values requested in the problem,  which is incorrect.

2) The expression a(k)-1 is never negative.  It's lowest value is the limit at x--> +infinity at 0.