1、4/(a+1)(a-1)-2/a(a+1)
=4a/a(a+1)(a-1)-2(a-1)/a(a+1)(a-1)
=(4a-2a+2)/a(a+1)(a-1)
=2(a+1)/a(a+1)(a-1)
=2/(a²+a)

2、=1/5(a-4)+5/(a-4)²
=(a-4)/5(a-4)²+25/5(a-4)²
=(a+21)/5(a-4)²

3、=x²/(x-1)-(x+1)(x-1)/(x-1)
=(x²-x²+1)/(x-1)
=1/(x-1)