lim(x->-1) [ 1/(x+1) -1/(x^3+1) ]=lim(x->-1) { 1/(x+1) -1/[(x+1)(x^2-1+1) ] }=lim(x->-1) (x^2-x+1-1)/[(x+1)(x^2-1+1)]=lim(x->-1) x(x-1)/[ (x+1)(x^2-1+1)]-> ∞
