4a^3/(a^4+x^4)+1/(a+x)+2a/(a^2+x^2)+1/(a-x) =4a^3/(a^4+x^4)+2a/(a^2+x^2)+[1/(a-x) )+1/(a+x）]=4a^3/(a^4+x^4)+[2a/(a^2+x^2)+2a/(a^2-x^2)]=4a^3/(a^4+x^4)+4a^3/(a^4-x^4)=8a^7/(a^8-x^8)