1. (a²/(a+1)-a³/(a²+2a+1)=a²/(a+1)-a³/(a+1)²=(a²*(a+1)-a³)/(a+1)²=
=(a³+a²-a³)/(a+1)²=a²/(a+1)².
2. a/(a+1)-a²/(a²-1)=a/(a+1)-a²/((a+1)*(a+1))=(a*(a-1)-a²)/(a²-1)=(a²-1-a²)/(a²-1)=
=-1/(a²-1).
3. (a²/(a+1)²):(-1/(a²-1)=a²*(a²-1):(-(a+1)²)=(a⁴-a²):(-(a+1)²)=((-3)⁴-(-3)²):(-(-3+1)²=
=(81-9):(-4)=72/(-4)=-18.