原式=[1/(1-x)+1/(1+x)]+2/(1+x^2)
=[(1+x+1-x)/(1-x)(1+x)]+2/(1+x^2)
=2/(1-x^2)+2/(1+x^2)
=2[(1+x^2)+(1-x^2)]/[(1+x^2)(1-x^2)]
=4/(1-x^4)