lim(x→-1) [3/(x^3+1)-1/(x+1)］=lim(x→-1) [3- (x^2-x+1) ]/[(x+1)(x^2-x+1) ]=lim(x→-1) -(x^2-x-2)/[(x+1)(x^2-x+1) ]=lim(x→-1) -(x+1)(x-2)/[(x+1)(x^2-x+1) ]=lim(x→-1) -(x-2)/(x^2-x+1) =-(-1-2)/(1+1+1)=1
=3
