[(a-1)/3a-(a+1)/a]×a/(a²-1)
=[(a-1)/3a-(a+1)/a]×a/(a-1)(a+1)
=1/3(a+1)-1/(a-1)
=(a-1)/3(a²-1)-3(a+1)/3(a²-1)
=(a-1-3a-3)/3(a²-1)
=(-2a-4)/(3a²-3)