(\frac{1}{m}-\frac{1}{n})*\frac{m^2n^2}{m^2-n^2}=\frac{n-m}{mn}*\frac{m^2n^2}{(m-n)(m+n)}=\frac{-mn}{m+n}
(\frac{1}{m}-\frac{1}{n})*\frac{m^2n^2}{m^2-n^2}=\frac{n-m}{mn}*\frac{m^2n^2}{(m-n)(m+n)}=\frac{-mn}{m+n}