a=1/(x+1)+(-1)/(x-1)
=(x-1)/(x-1)(x+1)-(x+1)/(x-1)(x+1)
=[(x-1)-(x+1)]/(x-1)(x+1)
=-2/(x-1)(x+1)
=-2/(x²-1）
又∵b=-2/(x²-1）
∴ab的关系是a=b