Решите, плиз. 22 балла!

$( \frac{3m-n}{3mn+ m^{2} } - \frac{3m+n}{3mn- m^{2} } ): \frac{4(m^{2} +n^{2}) }{ m^{2}-9n^{2}}= \\ \\ ( \frac{3m-n}{m(3n+m)} - \frac{3m+n}{m(3n-m)}): \frac{4(m^{2} +n^{2})}{(m-3n)(m+3n)}= \\ \\ \frac{(3m-n)(3n-m)-(3m+n)(3n+m)}{m(3n+m)(3n-m)} : \frac{4(m^{2} +n^{2})}{(m-3n)(m+3n)}= \\ \\ \frac{9mn-3m^{2}-3n^{2}+mn-9mn-3m^{2}-3n^{2} -mn }{m(3n+m)(3n-m)} * \frac{-(3n-m)(m+3n)}{4(m^{2} +n^{2})}= \\ \\ - \frac{-6m^{2}-6n^{2} }{4m(m^{2} +n^{2})} =[tex] \frac{6(m^{2} +n^{2})}{4m(m^{2} +n^{2})} = \frac{3}{2m}$

.
$[tex](\frac{my}{m-y} +m)* \frac{ m^{2}-n^{2} }{my+y^{2} } * \frac{m^{2}-y^{2}}{m+n}- \frac{ m^{2}-mn+n^{2} }{m^{3} +n^{2} } + \frac{1}{m+n}= \\ \\ \frac{my+m(m-n)}{m-n} * \frac{(m-n)(m+n)}{y(m+y)}* \frac{(m-y)(m+y)}{m+n} - \frac{m^{2}-mn+n^{2}}{(m+n)( m^{2}-mn+n^{2}) } + \frac{1}{m+n}= \\ \\ \frac{m^{2} (m-n)}{y} - \frac{1}{m+n} + \frac{1}{m+n} =\frac{m^{2} (m-n)}{y}$