1)(a-1)/(a+5)(a-7)-(a-7)/(a+5)(a-1)=[(a-1)²-(a-7)²]/(a+5)(a-7)(a-1)=
=(a-1-a+7)(a-1+a-7)/(a+5)(a-7)(a-1)=6(2a-8)/(a+5)(a-7)(a-1)=
=12(a-4)/(a+5)(a-7)(a-1)
2)12(a-4)/(a+5)(a-7)(a-1) : 2(a-4)/(a+5)(a-1)=
=12(a-4)/(a+5)(a-7)(a-1) * (a+5)(a-1)/2(a-4)=6/(a-7)
3)6/(a-7)-(a-1)/(a-7)=(6-a+1)/(a-7)=(7-a)/(a-7)=-1