(m-n)/(2m+2n)-(m²+n²)/(m²-n²)
=(m-n)/[2(m+n)]-(m²+n²)/[(m+n)(m-n)]
=[(m-n)²-2(m²+n²)]/[2(m+n)(m-n)]
=(m²-2mn+n²-2m²-2n²)/[2(m+n)(m-n)]
=-(m²+2mn+n²)/[2(m+n)(m-n)]
=-(m+n)²/[2(m+n)(m-n)]
=-(m+n)/[2(m-n)]