Question

先化简，再求值:(1/m-1/n)÷m²-2mn+n²/mn，其中m=-3，n=5

Answer 1

[(n-m)/mn]/(m-n)^2/mn=-1/m-n=1/8

Answer 2

(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