令2+2/(a-1)-2a/(a^2-1)=0,通分得[2(a^2-1)+2(a+1)-2a]/(a^2-1)=0,化简得,2a^2/(a^2-1)=0,即2a^2=0,a^2-1≠0,解得a=0
所以,当a=0时分式的值为0