[(n-m)/mn]/(m-n)^2/mn=-1/m-n=1/8
(1/m-1/n)=(n-m)/(mn)(m^2-2mn+n^2)/(mn)=(m-n)^2/(mn)(1/m-1/n)÷[ (m²-2mn+n²)/mn ]=[(n-m)/(mn)] / [(m-n)^2/(mn)]=-1/(m-n)=-1/(-3-5)=1/8
