BiNOMIAL ExPANSION Of NeGATIVE InDEX

Why in binomial expansion of negative index we have x<1 ? as (1+x)^1 = 1x+x^2x^3... turns out to be true for x=1 [Grandi's Series] .

@eiπ
It is something related to convergent and divergent series which is far beyond jee syllabus