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