Ответ:

а) \(\frac{m(x-y)(x+y)}{2(m+4)}\cdot\frac{3(m+4)}{m(x+y)}=\frac{3(x-y)}2\).

б) \(\frac{a(x+y)}{(x-y)^2}\cdot\frac{x(x-y)}{7(x+y)}=\frac{ax}{7(x-y)}\).

в) \(\frac{(x-y)(x^2+xy+y^2)}{x+y}\cdot\frac{(x-y)(x+y)}{x^2+xy+y^2}=(x-y)^2\).

г) \(\frac{(a-1)(a+1)}{(a+1)(a^2-a+1)}\cdot\frac{a^2-a+1}{(a+1)^2}=\frac{a-1}{(a+1)^2}\).

Ответ: а) \(\frac{3(x-y)}2\); б) \(\frac{ax}{7(x-y)}\); в) \((x-y)^2\); г) \(\frac{a-1}{(a+1)^2}\).