1) (a+1)/(a-1)-4a/(a²-1)=(a+1)/(a-1)-4a/((a-1)(a+1))=((a+1)²-4a))/(a²-1)=
=(a²+2a+1-4a)/(a²-1)=(a²-2a+1)/(a²-1)=(a-1)²/((a-1)(a+1))=(a-1)/(a+1).
2) (a-1)/(a²+a)=(a-1)/(a*(a+1)).
3) (1):(2)=a.
=(72/24)*a⁽⁷⁻³⁾*b⁽¹⁶⁻¹⁶⁾/c⁽¹⁰⁺⁸⁾=3*a⁴b⁰/c¹⁸=3a⁴/c¹⁸=3a⁴c⁻¹⁸.