1) ОДЗ: a≠–7; a(x+7)/a(x+7)=[a(x+7)]:(x+7)/[a(x+7)]:(x+7)=a/7;
2) ОДЗ: k≠4; 9(4–k)/(k–4)=–9(k–4)/(k–4)=[–9(k–4)]:(k–4)/(k–4):(k–4)=9/1=9;
3) ОДЗ: ab≠0, m≠2; 20a²(m–2)/ab(2–m)=–20a²(2–m)/ab(2–m)=–20a²(2–m):(2–m)/ab(2–m):(2–m)=20a²/ab=a•20a/a•b=(a•20a):a/(a•b):a=20a/b;