2.
х(х+а)-3(х+а) х(х+4)-а(х+4)
=-----------------×---------------------=
х(х-а)-3(х-а) х(х+4)+а(х+4)
(х+а)(х-3)(х-а)(х+4)
=----------------------------=1.
(х-3)(х-а)(х+а)(х+4)
4.
m(m+1)-n(m+1) m(m-1)-n(m-1)
=----------------------×--------------------------=
m(m+1)+n(m+1) m(m-1)+n(m-1)
(m-n)(m+1)(m-n)(m-1)
=----------------------------------=
(m+1)(m+n)(m+n)(m-1)
( m-n)^2
=-----------
(m+n)^2