1/(x+1)-3/(x^3+1)=(x^2-x+1)/(x^3+1)-3/(x^3+1)=(x^2-x-2)/(x^3+1)=(x+1)(x-2)/(x^3+1)=(x-2)/(x^2-x+1)x趋于1，等于(1-2)/(1-1+1)=-1
